Archaeologists working with the Ministry of Defence, which is building hundreds of military homes at Larkhill, made the discovery.
The site was used to emulate conditions soldiers would face in trenches in France and Belgium during the war.
More than 100 pieces of graffiti have been uncovered in the chalk walls of the trenches and tunnels.
Soldiers training on the site who left their names include decorated heroes and one recorded deserter.
The names feature Wiltshire men as well as West Yorkshire coal miners, and two brothers wrote "Semper Fidelis" (Ever Faithful) beneath their signatures.
Archaeologist Si Cleggett, who described the Larkhill excavation as "unique", said:  "It has been a humbling experience to stand and read the names of young soldiers in the very spaces they occupied before leaving for war.
"Having stood in their footprints a century after their time at Larkhill, we really will remember them."
Training relics such as grenades, ammunition and food tins were also found.
The Australian 3rd Division infantry trained on Salisbury Plain in 1916 and the dig uncovered a chalk plaque inscribed with the names of Australian bombers - soldiers specially trained to use hand grenades.
One name is Private Lawrence Carthage Weathers, who won the Victoria Cross in September 1918 for attacking a machine gun post with grenades, capturing it and taking 180 prisoners.
Steve White, of building contractors GABLE, said the Larkhill tunnels project "represented a unique opportunity to be a part of a story of unprecedented archaeological significance".
About 4,300 soldiers and their families are to move to Wiltshire from Germany and UK bases by 2019 as part of the Army Basing Plan.

On Wednesday, shares of Chinese solar power manufacturer Hanergy Thin Film were suspended after falling 47%, wiping out $18.6bn (Â£11.9bn) from its market capitalisation.
That was followed by shares of Goldin Financial, which fell 55% on Thursday.
Both firms had seen the price of their shares surge over the past year.
Shares of Goldin Financial, which is 70%-owned by Hong Kong billionaire Pan Sutong, have jumped by more than 400% between September 2014 and March this year, according to Reuters.
The firm had said that it was not aware of the reason behind the big jump in its share price.
About half of Goldin's value - $16.12bn - was erased on Thursday as shares fell.
Meanwhile, Hanergy shares had risen five-fold since September before the sell-off began.
The firm, controlled by Chinese billionaire Li Hejun, had issued a statement to the Hong Kong Stock Exchange on Wednesday saying its shares had been halted from trade "pending the release of an announcement containing inside information".
Li has been considered one of China's richest men on the back of his stake in Hanergy.
Nicholas Teo, market analyst at trading firm CMC markets said he had noticed the dramatic drop in Hanergy's shares right away because it had everyone "scratching their heads".
"The company was very quick to call a halt to the trading, but without any news, there is plenty of speculation and potentially room for further reaction to the downside if the trading suspension is lifted," he said.
"Even after the drop though, Hanergy's shares are basically twice as expensive as some of the firms in that sector."
David Kuo, chief executive of The Motley Fool Singapore backed the view that Hanergy shares were overvalued.
"Hanergy is involved in the much-hyped solar-panel industry. Investors are paying $50 for every dollar of profit the company makes," he said.
"When expectation overtakes reality, reality eventually wins. The market is experiencing a dose of reality."
There has been speculation as to the reason why the Chinese company halted trading, with Reuters citing an unnamed source as saying the firm is under investigation by Hong Kong regulators.
Hong Kong's Securities and Futures Commission (SFC) has declined to make a comment.
Local reports also said that Li, chairman of the firm, did not attend the firm's annual general meeting in Hong Kong on Wednesday.

Crews were called to the incident at about 17:00 on Friday.
Five appliances were initially sent to the scene with one remaining on site on Saturday afternoon.
No-one was injured in the incident.
A "unique" network of tunnels used to train soldiers to fight in World War One has been found on Salisbury Plain.
[NEXT_CONCEPT]
Shares of two Hong Kong-listed companies have plunged by about 50% in the past two days, surprising market watchers across the region.
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Firefighters have been tackling a blaze at a factory on the Isle of Scalpay in the Western Isles.