In an open, entertaining game in Nanjing, it was the Hammers who opened the scoring when Angelino hand-balled on the ground and Mark Noble converted.
But two goals in two minutes – the first a sublime piece of control and cool finish from skipper David Silva on 31 minutes– and the second a penalty that was won and converted by Lukas Nmecha on 33.
Raheem Sterling added a third just before the hour-mark and completed the rout on 72.
City looked fresh and full of vim and vigour despite the humidity and the fact this was the first game of pre-season.
As ever, David Silva pulled the strings in a side that was a mixture of youth and experience
West Ham fielded a strong XI and caused plenty of problems, with Felipe Anderson a constant menace on the right flank.
And it was Anderson’s trickery that felled Angelino in the box, and he was unfortunate to drag the ball with his arm as he lay on the turf.
Noble scored from the spot, but the Blues hit back and hit hard, with Silva levelling and Nmecha stroking the ball home from the spot.
Ben Johnson had legitimate claims for a second penalty shortly after but referee Craig Pawson felt otherwise.
The Blues brought on more first-team experience after the break and two of them combined superbly to put the game out of West Ham’s reach.
As the ball came to Leroy Sane, the German winger sliced the Hammers’ defence apart with a sumptuous through pass that Sterling raced on to and calmly placed it past Roberto.
City added a fourth on 72 when De Bruyne charged down a West Ham clearance, Sane collected the loose ball before finding De Bruyne who passed it on to Sterling who made no mistake from close range.
There could have been more goals, but as a first outing of the summer, this was impressive stuff.
Moment of the match
City were trailing and at risk of falling further behind when teenage winger Adrian Bernabe collected a pass, looked up and lofted a sublime cross into the path of Silva who chested the cross before gently volleying past the keeper.
It was the moment that axis swung firmly back in City’s favour with the Hammers eventually swept aside.
Image of the day
City bossed the second half, but the overall possession was down on a normal Premier League performance with 55% to 45% in the Blues’ favour.
City had 10 shots on target compared to West Ham’s five.
There was no VAR in the game – if there had been, both sides could well have had at least another spot-kick each.
Man of the match: Tommy Doyle
The Academy graduate proved he is ready to challenge for senior football with a performance that had a bit of everything.
Tenacious, excellent vision and passing and a work ethic that his granddad and former City legend Mike would have been extremely proud of.
Not bad for a 17-year-old!
What it means...
City go into the Premier League Asia Trophy final against Wolves in Shanghai.
Wolves thrashed Newcastle United 4-0 in Nanjing prior to the Blues’ game.
The match will be played on Saturday, kick-off 12.30pm BST.