The Olympic champion clocked three mins 42.58 seconds with Britain's James Guy taking silver at 1.17secs back and Canada's Ryan Cochrane won bronze at 2.01 secs as the first medals of these championships were decided in Kazan.
This is the first of four individual events Sun, the world 1500m freestyle record holder, will race in Kazan as well as the 200m, 800m and the mile distance.