Nicky Maynard scored twice as Bristol City ended a sequence of eight games without a victory.
Maynard struck from the corner of the six-yard box after 23 seconds and doubled his tally after Scott Loach parried a shot from Ivan Sproule.
Grzegorz Rasiak smashed home Jobi McAnuff's cross to pull a goal back.
Marvin Elliott restored City's two-goal lead with a 25-yard strike but then headed into his own net before Dele Adebola headed City's fourth late on.
Watford manager Brendan Rodgers:
"When you give a team like Bristol City a two-goal start it becomes very, very difficult and I was really disappointed at half-time.
"Any person that's worked with me knows it's about passion, commitment and desire. We didn't have that in the first half.
"In the second half we were a bit better, in terms of at least we came out with fire in our belly, we got the goal back for 2-1, then it looked like we might then have the momentum to go and play."
Bristol City manager Gary Johnson: "I thought it was a six-pointer. I said that before the game to the lads as well because it was an important three points for Watford.
"It's a stage of the season now where you don't want to be looking over your shoulder thinking you're competing with relegation, and we felt that two or three wins on the trot would put us back where we enjoy competing more, in the top half of the table.
"We hadn't played badly in the last two or three weeks but we hadn't picked up the wins."
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