Pompey forward Matt Mayes netted a hat-trick as the young Glovers were hit for five on the south coast.
Fiftteen-year-old Devon Arnold netted for Town, however his goal proved to be in vain on a disappointing afternoon.
The hosts got off to a flying start when Mayes fired a volley into the top corner.
It stayed that way until the second period, which saw a change of intensity from the Hampshire side with James Whiting and Bradley Lethbridge grabbing two goals in five minutes to make it 3-0.
Arnold beat the Portsmouth backline to coolly slot into the corner to reduce the deficit, then Mayes further showed his goalscoring prowess.
He touched in from close range for his second and Portsmouth’s fourth before rounding off the scoring with a shot from distance in the final minute.