Half time at Vicarage Road sees Watford lead Peterborough thanks to an early goal from Troy Deeney.
The home side took the lead after just 10 minutes when Deeney headed in Mark Yeates's corner at the back post to make 1-0.
The Hornets continued to dominate possession at Vicarage Road, with another set piece from Yeates nearly headed home.
Barnsley caused problems from two corners on 20 minutes, forcing Manuel Almunia to keep them out.
Luke Steele pulled off a full-length save to deny Fernando Forestieri after 24 minutes.
Deeney was denied a second goal four minutes later after his effort was well blocked by the defence.
Marco Cassetti came close from the resulting corner, his header from the near post hitting the top of the crossbar.
Forestieri could have doubled Watford's lead after a defensive error let him in, but the Italian wasted the chance.
