The accident took place at around 8am on the outbound Thep Kasattree Road in Thalang district.
Tha Chatchai police said 18 people sustained slight injuries, while two others were in moderate condition. They were rushed to Thalang Hospital.
The bus was taking 16 Chinese tourists plus a tour group leader, as well as a Thai guide from a hotel in the heart of Phuket to a pier to board a boat to Similan Island in Phang Nga. The other two Thais injured were the driver and his assistant.
Police said the bus lost control at a sharp curve in the road and fell to the right side.
Phuket’s tourist police chief Pol Colonel Ekchai Siri and officials from the tourist assistance centre rushed to the hospital to help the tourists. The officials have also informed the Chinese consulate in Phuket to visit the injured and provide help.