There's no official car park but if you come off the A27 onto A2030 (Eastern Road) you will eventualy come to a crossroads (you'll see a phone shop with a big clock phone on the front and big yellow storage) you can then go straight over which is Rodney Road and there will be signs for car parks. Or you can park at Milton Cross school which is on Milton Road. You take the same route but when you get into Rodney Road you go to the mini round about and double back on yourself and then left onto Milton Road, the school is then on the right.
There is a car park at Fratton Park but that's not been in use recently, although it's just changed hands. There's also a large car park at B&Q right opposite FP but I would guess you'd get a parking ticket there.
Other option is to use one of the side streets off Eastern Road or anywhere near the ground as there's little in the way of parking restrictions.