Two goals from Daniel Sturridge, and one each from Florent Malouda, Frank Lampard and own goal from John Eustace saw the West Londoners through with plenty to spare.
Sturridge, 20, took just five minutes to open his Chelsea account following his move last season from Manchester City.
Lampard's deep left-sided corner was headed back across goal by Alex. John Terry took a swipe at goal but the ball screwed into the grateful path of Sturridge, who tucked home from four yards past goalkeeper Scott Loach.
Yuri Zhirkov, Lampard and Malouda all had chances soon after to extend the lead and within more nine minutes Watford's Eustace knocked than ball into his own net after confusion in their defence, caused by Joe Cole, saw the ball rebound off Adrian Mariappa and Loach before the hapless midfielder got the last touch and the ball crossed the line.
Poor Watford then conceded a third on 21 minutes. A long-range shot by Zhirkov was deflected by Malouda which sent Loach diving the wrong way.
Shooting practice by the Premier League leaders continued with Lampard and Zhirkov peppering the opposition goal without success.
Sturridge had a good chance on 38 minutes to net his second after excellent set-up play saw the striker in the Watford penalty area, but he drove a low left-footer just wide of Loach's right post.
Watford started brightest in the second half and almost scored on 51 minutes through American Jay Demerit's glancing header from six yards out, but Chelsea goalkeeper Hilario was perfectly placed to smother the chance.
But class told and on 63 minutes up popped Lampard to arrow home from 20 yards an unstoppable, inch-perfect shot - high and wide of Loach's despairing dive to make it four.
Number five came four minutes later and it fell to Sturridge, who collected Ashley Cole's byline cross and in one swift movement six yards out, took control of the ball and flicked the ball with the outside of his right boot past Loach.