Olympic champion in London last year at 400 and 1,500 metres, Sun touched in a time of seven minutes 41.36 seconds, with American Michael McBroom taking silver in 7:43.60 and Ryan Cochrane of Canada bronze in 7:43.70.
Sun is only the second man after Australian Grant Hackett to win world titles at 400, 800 and 1,500 metres.
His titles at the three distances have been earned over two different championships - Sun won the 800 and 1,500 in Shanghai two years ago and was second in the 400 - but he remains on course to match Hackett's three golds from the 2005 edition in Montreal.
The 1,500 metres freestyle final is on the final day of the championships on Sunday.
(Reporting by Iain Rogers, editing by Pritha Sarkar)
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