Based on the combination strategy of dual spatial grids，a high-efficiency body-fitting DSMC （direct simulation Monte Carlo） method was developed.By mapping the coordinate of the simulated molecule in structural grid in the physical space to the Cartesian grid in the calculation space，and by completing the location of the grid unit to which the molecule belongs and the judgment of whether the molecule and the boundary are interacting in the calculation space，the body-fitness and computational efficiency of the DSMC method were both improved，in combination with the advantages of the body-fitness of the structured grid and the high-efficiency calculation of the Cartesian grid.Based on the combination strategy of dual spatial grids，DSMC programs were established through direct and indirect mapping methods to simulate the gas flow through the microscale contraction expansion nozzle and the supersonic flow past circular cylinder.Numerical results showed that the two methods were effective in simulating the viscous effect and the velocity slip phenomenon due to the shrinkage of the micro nozzle and the shock wave phenomenon caused by the supersonic flow past circular cylinder，demonstrating the effectiveness of the methods.Compared with the traditional structural grid method，the calculation efficiency of the two methods was respectively increased by 3.85 times and 2.85 times on average，showing the high efficiency of the methods.