Gavin Mahon the Watford captain had the first chance after thee minutes from the middle of the penalty area he hit 15-yard left-foot shot and it was tipped over the bar by Carlo Nash the Preston keeper for right-wing corner.
Matthew Spring made it 1-0 to Watford after just ten minutes when then ex-Leeds £150,000 signing hit a 15-yard left-foot shot into the bottom left-hand corner of the net from the middle of the penalty area from a Mahon right-foot pass inside the right channel.
Preston pulled a goal back straight away when Callum Davidson, the Scottish international, added a second goal of the season to his goal on Tuesday night - his first goal for 14 months.
From a central position he hooked home a seven-yard left-foot shot into the middle of the net after David Nugent had headed down Graham Alexander's right-foot, right-wing cross.
Watford skipper Mahon was booked for a foul on Preston's on-loan midfielder David Jones.
Early in the second half Jay DeMerit was booked for foul on England Under-21's striker Nugent.
Daniele Dichio, without a goal in the league this season in 16 games, came on as subsitute for Patrick Agyemang with 20 minutes to go and then Lewis Neal replaced Matt Hill.
Watford goalscorer Matthew Spring was booked for foul on Brian O'Neil after 74 minutes and substitute Dichio was booked two minutes later for a foul on DeMerit.
Malky Mackay had a header cleared off the line by Alexander with 12 minutes to go from an Ashley Young left-wing corner.