For the simulation of thermochemical non-equilibrium gas flows by direct simulation Monte Carlo(DSMC), a model based on maximum entropy (ME) is developed to calculate reaction probability and post reaction energy disposal. For non-reacting collisions, a generalized Larsen-Borgnakke method is employed for energy exchange. For reacting collisions, ME method is employed for energy exchange and chemical reaction. Hypersonic gas flow around a cylinder in high altitude and gas flow around a blunted cone of low density and high entropy are simulated and compared with results of DSMC method and HEG wind-tunnel data. The DSMC-ME is demonstrated reliable.