This paper deals with the mechanism of molecular in elastic collision，presents the probability selective models of molecular internal excitation and molecular reactions，and introduces the energy transfer models among translational energy，rotational energy and vibrational energy. The results of the stagnation streamline flow of a hypersonic blunt body obtained by DSMC method conform with the results obtained by viscous shock layer (VSL) equation method at the lower altitude，which shows that models used in the DSMC simulation are appropriate.