Chinnapasaen qualified for the men's 50 m freestyle at the 2004 Summer Olympics in Athens, by posting a FINA B-standard entry time of 23.20 from the Southeast Asian Games in Hanoi, Vietnam. He challenged seven other swimmers on the sixth heat, including three-time Olympian Julio Santos of Ecuador. He raced to sixth place by one hundredth of a second (0.01) behind Cyprus' Chrysanthos Papachrysanthou in 23.52. Chinnapasaen failed to advance into the semifinals, as he matched a forty-sixth place tie with SEA Games medalist and former rival Allen Ong of Malaysia in the preliminaries.