Abstract: Material outgassing is a common phenomenon in vacuum thermal test. In this paper, the course and mechanism of material outgassing are analyzed, and two methods of calculating outgassing amount are proposed. For a gradual outgassing course, the gas consistence distribution for a flat board model and the gas amount formula are derived according to the Second Fick Law for gas diffusion. For an acute outgassing course in a short time, a fitting curve is obtained according to the pumping data after outgassing, so the outgassing amount is obtained from the curve. Furthermore, the course of thermal conduction caused by material outgassing is analyzed as a theoretical basis to solve related problems in vacuum thermal test.