Portsmouth failed to pick up their first away win of the season in Michael Appleton's first game in charge as Watford eased to victory.
On loan winger Michael Kightly put the home side ahead after just 75 seconds when his opportunist shot was deflected past Stephen Henderson in the Pompey goal.
The Hornets continued to dominate and doubled the lead just before the half-time break.
Marvin Sordell peeled away towards the back post and volleyed Mark Yeates's corner home from ten yards.
Sean Dyche's side jump away from the relegation zone to 19th and lie level with Pompey on 19 points.
