Abstract: To solve the problem of the simulation of gas flows with large density variation,the multi-region variable time step method was implemented.A remaining time scaling process should be performed for the molecules passing through the interface between two regions with different time steps,thus the continuity of the macroscopic flux across the interface could be maintained.An efficient multi-region interface and surface processing algorithm was presented.It prescribed that each interface belonged to only one region,and the wall surface spanning multi-region was split into a plurality of wall surfaces,each of them belonged to only one region.A simulation molecule could only interact with the interface and wall surface of the region the simulation molecule belonged to.The simulation results of a nozzle flow show that this method can significantly improve the computation efficiency/