The trial mining of gas hydrate is the only way which must be passed from the theoretical research to commercial exploitation. The steam mining is a combine mining method. Based on the pressure relief mining, hot steam is injected to the hole for thermal excitation in the gas hydrate target layer, which can avoid “self-preservation effect”, promote further decomposition of the hydrate and expand mining area theoretically. Numerical calculation was done on the dynamic process of steam heating hydrate layer by FLUENT software. Through the comparative analysis on the simulation results, it was decided that the best steam power was 20kW and the heat injection time was 38 hours in order to meet the requirements of the premise mining. In the natural gas hydrate production project in Muli basin of Qinghai, the steam mining lasted for 5.2 hours with the gas production of 3.25m3.