In August 2018, Sun competed in every freestyle event from the 200 to the 1500 metre freestyle at the Asian Games. He also competed in the 4 × 100 and 4 × 200 metre freestyle relay events. In the 200 metre freestyle, Sun won gold with a time of 1:45.43. He finished 1st in the 400 metre freestyle with a time of 3:42.92, over four seconds ahead of runner-up Naito Ehara. By beating Horton's time of 3:43.76 from the 2018 Commonwealth Games, Sun became the number-one swimmer in the world for that event for 2018. In the men's 800 metre freestyle, which was introduced to the Asian Games for the first time, he won gold with a time of 7:48.36. In the 1500 metre freestyle, he finished 1st with a time of 14:58.53, beating runner-up Nguyễn Huy Hoàng by over three seconds. It was the first time Sun swam the event within 15 minutes since the 2014 Asian Games, and his third consecutive Asian Games title in the 1500 metre freestyle. Representing China, Sun helped his team finish second in the 4 × 100 metre freestyle relay with an overall time of 3:13.29, behind Japan's time of 3:12.68. In the 4 × 200 metre freestyle relay, he helped China win silver with an overall time of 7:05.45, behind Japan's 7:05.17.