The changeable, atrocious environment of stratosphere affects the capability and performance of the structure. An effective and economical approach used to analyze thermal state of stratospheric aerostat was investigated in this study. A simplified thermal model was built in environmental experiment which simulates stratosphere and takes radiation of the sun, air and earth. This model solved a coupling problem of heat conduction, heat convection and radiant heat transfer and obtained a pleasant result through numerical simulations of Fluent. The whole process makes great effect to further analysis and study of stratospheric structure.