The base will be home to HMS Queen Elizabeth and HMS Prince of Wales, the largest ships ever built for the navy.
Work on Portsmouth's jetty, built in the 1920s and last upgraded in the 1970s, has begun, with upgrades expected to be completed next autumn.
It forms part of a Â£100m infrastructure package to prepare the base for the arrival of the first carrier.
HMS Queen Elizabeth measures 280m (300 yards) long and can travel up to 500 miles (800km) a day.
The ship's systems are being tested and it will conduct sea trials next year before being handed over to the Royal Navy.
It is expected to come into service in 2020.
HMS Prince of Wales is being constructed at shipyards around the UK before it is assembled at Rosyth.
Once completed, the revamped naval base is expected to create hundreds of jobs in Portsmouth.
In November 2013, BAE Systems announced it was ending shipbuilding in Portsmouth, leading to 585 voluntary redundancies, 160 compulsory redundancies, and the redeployment of 175 members of staff.

The CPS Inspectorate's report said the CPS was slow to contact victims in almost half of the cases it examined and sent letters that "lacked empathy".
In a majority of cases there was also no evidence that victims' views were taken into account on key decisions.
The CPS said its own data showed a "markedly different" picture to the "small sample" used in the report.
Her Majesty's Crown Prosecution Service Inspectorate's communicating with victims report looked at the speed and quality of the CPS's communication with victims of crime.
As part of its investigation, inspectors visited six of the 13 CPS areas in England and Wales, during a three-week period. They examined 162 case and complaint files and interviewed CPS staff, police and crime commissioners and partner organisations such as Citizens Advice.
The Inspectorate said the CPS should be more consistent in the quality of their letters and how quickly they send them to victims, and added the CPS and police should agree on their roles in communicating case outcomes to victims to avoid sending confusing or duplicated information.
They also stated the CPS should "implement an effective process" for notifying victim liaison units when charges are altered in a case, so that staff can inform victims.
Inspectors did say the CPS was facing increased pressure due to staff cuts, and the report also highlighted that the organisation had taken steps to improve service, including drafting new guidance about speaking to witnesses at court to improve the quality of service.
But the CPS criticised the small sample used in the report and said their own figures showed they had improved service.
Chief crown prosecutor Martin Goldman said the CPS is "committed to delivering a high quality service to victims and witnesses and supporting them throughout their case".
He said: "Last year a survey showed that two-thirds of victims and three-quarters of witnesses were satisfied or very satisfied with the service they received from the CPS.
"80% of letters to victims due to reach them within one day meet the target, and this increases to 89% of letters due to reach victims within five days."

The man, 31, approached Police Community Support Officers guarding a cordon in Albert Road in Plymouth on Thursday evening.
Devon and Cornwall Police said it was believed he was assaulted in nearby Exmouth Road.
Two men have been arrested in connection with the stabbing.
More on this and the latest on what's happening in Devon here
A force spokesman said the injured man was in a "critical condition" in Derriford Hospital.
Two men, aged 58 and 46, were arrested on suspicion of assault causing grievous bodily harm with intent. They are awaiting questioning.
The Royal Navy has released an image of planned improvements to a jetty at Portsmouth Naval Base.
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Victims of crime are "let down" by poor communication from the Crown Prosecution Service, a watchdog claims.
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A stabbing victim walked a third of a mile (480m) to the scene of a fire to find help, police have said.