Thanckes Park on the banks of the River Tamar will be the venue for the free event, which starts at 11am with a veteran's parade.
The park will be divided into three zones: a military zone with hands-on experiences and stands; an arena for drills and displays; and a stage, where dancers and singers will be performing.
The event is a chance for members of the public to show their support for the men and women of the UK's armed forces.
It will also offer the opportunity to meet serving military personnel and see some of the hardware and equipment they use.
Visitors will be able to get a close look at a Royal Marine Jackal armoured vehicle, sit inside an RAF glider simulator and try their hand at firefighting in an interactive display provided by HMS Raleigh's Maritime Survival School.
The Band of Her Majesty's Royal Marines Plymouth, which is based at HMS Raleigh in the town, will be performing twice.
And Royal Navy recruits from the training establishment will take centre stage in the arena with an arms drill display.
Other highlights of the arena programme will include: a one-off performance of the Cornish All Schools Choir, featuring children from the local area; the Carbeile School Field Gun Crew in action: and the Ministry of Defence Police Dog Unit showcasing its skills.
From 5.30pm, activity will move to the stage where a programme has been put together featuring Cornish musical talent. After headline act Company B has got the crowd moving, the evening will culminate with a firework display.
Free parking for Armed Forces Day is available at Antony House with an overflow car park at HMS Raleigh's Spickernell site.
A free shuttle bus service will take visitors to Thanckes Park for the event and then return them to their cars.