The heat transfer performance of vapor-condenser has been studied under different temperature of cold trap and different thickness of the frost layer in this paper. The relationship between dimensionless number Nu and Kn is obtained. The results show that the capturing efficiency of cold trap increases with the decrease of the surface temperature of vapor-condenser. The frost accumulated on the surface of vapor-condenser can cause the overall heat transfer coefficient decrease, which has a negative effect on heat transfer of vapor-condenser. Kn has an effect on the heat transfer of vapor-condenser during vacuum cooling.