The incident occurred at 8 a.m. in Thalang District as the bus was heading from Phuket to a pier where the tourists planned to get on a boat to Similan Island in Phang Nga Province.
Suddenly, the driver lost control at a curve, causing the bus to overturn, Nation Thailand reported citing local police.
The bus was carrying 16 Chinese tourists, a tour group leader, as well as a Thai local guide. The other two Thais injured were the driver and his assistant.
Most sustained very slight injuries, while two were in moderate condition, Bangkok Post reported.
Police are expanding the investigation to consider legal charges against the bus driver.
Thailand received 3.51 million Chinese tourists out of 28 million foreign arrivals. This year it targets 8 million Chinese visitors.