Abstract: To determine the temperature difference between the front side and the back side of a solar cell panel and the temperature variation during its temperature falling process, this paper employs a thermo-physical model of the solar cell panel in thermal vacuum tests. According to the thickness, the back-emissivity, the equivalent longitudinal conductivity factor of the panel, and its front temperature at a high temperature balance as well as the heat sink temperature, the temperature difference between the panel’s back side and the front-back is obtained. With the lumped parameter method, the expression of the time taken for the panel’s temperature falling and the variation of the temperature against time during cooling are derived, which may be used to predict the time taken for the panel’s temperature falling approximately. The analysis result can be used for the thermal analysis and forecasting the temperature falling process of solar cell panels, and for controlling the low temperature point in thermal vacuum tests.