Numerical simulations are presented about the effects of gas rarefaction on hypersonic flow field.Due to the extremely difficult experiment,limited wind-tunnel conditions and high cost,most problems in rarefied flow regime are investigated through numerical methods,in which the direct simulation Monte-Carlo(DSMC)method is widely adopted.And the unstructured DSMC method is employed here.Flows around a vertical plate at a given velocity 7 500 m/s are simulated.For gas rarefaction is judged by the free-stream Knudsen number(Kn),two vital factors are considered:molecular number density and the plate′s length.Cases in which Kn varies from 0.035 to13.36 are simulated.Flow characters in the whole rarefied regime are described,and flow-field structure affected by Knis analyzed.Then,the dimensionless position D*of a certain velocity in the stagnation line is chosen as the marker of flow field to measure its variation.Through flow-field tracing and least-square numerical method analyzing,it is proved that hypersonic rarefied flow field expands outward linearly with the increase of 1/2Kn.An empirical method is proposed,which can be used for the prediction of the hypersonic flow-field structure at a given inflow velocity,especially the shock wave position. Numerical simulations are presented about the effects of gas rarefaction on hypersonic flow field. Due to the extremely difficult experiment, limited wind-tunnel conditions and high cost, most problems in rarefied flow regime are investigated through numerical methods, in which the direct simulation Monte- The Carlo (DSMC) method is widely adopted. And the unstructured DSMC method is employed here. Flows around a vertical plate at a given velocity 7 500 m / s are simulated. For gas rarefaction is judged by the free-stream Knudsen number (Kn) , two vital factors are considered: molecular number density and the plate’s length. Chambers in which Kn varies from 0.035 to 13.36 are simulated. Flow characters in the whole rarefied regime are described, and flow-field structure affected by Knis analyzed. Then, the dimensionless position D * of a certain velocity in the stagnation line is chosen as the marker of flow field to measure its variation. Through flow-field tracing and least-square numerical method analyzing, it is proved that hypersonic rarefied flow field expands outwardly linearly with the increase of 1 / 2Kn. An empirical method is proposed, which can be used for the prediction of the hypersonic flow-field structure at a given inflow velocity, especially the shock wave position.