To develop and construct island, sea sand had been solidified into construction materials with flexural strength more than 20MPa under hydrothermal conditions by simulating diagenetic mechanism of underground. The experimental results show that the development in strength is mainly due to the formation of tobermorite and calcium silicate hydrate（C S H）. Ca（OH）2 content, curing time and curing temperature influence formation of tobermorite. The chloride ion dissolving out content of the sea sand from solidified sample decreases significantly after hydrothermal solidification, which is far less than regulation made by Ministry of Construction of China.