Numerical studies on effects of free stream on flowfield and performance of aerospike nozzle were presented. Aerospike nozzle performance losses and pressure distribution along plug surface were computed and compared at four different altitudes with typical free stream Mach numbers. It is shown that aerospike nozzle performance losses caused by free stream primarily result from increase of lift body base drag and decrease of plug pressure distribution. At mid and low altitudes, thrust of lift body base and plug ramp decreases with increasing free stream Mach number. The effects of free stream on performance decrease with increasing altitude, and from a certain altitude lower than design point thrust of plug ramp is not influenced by the free stream any more. For subsonic free stream, performance losses almost linearly increase with increasing Mach number. For transonic free stream, sudden performance degradation occurs. For the further supersonic range, performance losses increase slightly with increasing Mach number.