Soi Sim is an earthen island 400 m away from Titop island and 9-10 km from the Tuan Chau Tourist Port. With the area of 8.7 hectare, the island has very special geological formation in which 2/3 covered by Feralit and soil weathered by wind. It also includes a primitive forest with varieties of flora and fauna, and plants endemic to Halong Bay Ecology.
The island has been existed about hundred of million years ago and surrounded by the sea then there are endemic plants, according to researchers.
The name of Soi Sim derived from the ancient story that there were a lot of tomentose rose myrtle alive at the island then the local people called it Soi Sim.