This article mainly researched the influence of sidewall on binary external-compression inlet flow field.The three-dimensional flow fields of two inlet cases,with/without sidewall,were studied with the commercial CFD（computational fluid dynamics） software FLUENT.The performances of the case without sidewall were much worse than the case with sidewall on a comparative basis.Therefore,the performances of the inlet with sidewall between Mach number 2.0～3.0 were studied.As compared with the inlet's two-dimensional flow fields' performances,the three-dimensional flow fields' performances declined once the compressed air passed the sidewall and leaked out.After the analysis of the leakage flow under different Mach numbers,the influence law of leakage from sidewall was induced.