The inflatable cover was used in the low-temperature blackbody radiation source; the function of the external gas separation had great influences on blackbody radiation source performances. This paper presents a new way for performing numerical simulation for the three-dimensional inner flow field analysis of new inflatable cover and blackbody radiation source cavity by using the FLUENT software of CFD. By comparison, the simulation results and the acetone tracer particle experiments revealed the reason of the outside air into the cavity caused frost and dew, so we proposed a number of methods to reduce air convection inside and outside of the cavity for the using inflatable cover, and it gave theoretical basis guidance for the next optimal design of the new inflatable cover.