In order to understand the influence of ambient pressure on aerospike nozzle base pressure, the implicit NND(non-oscillation, no free coefficients dissipative) scheme was employed to solve 2-D laminar N-S equations. Aerospike nozzle flow fields under different ��C��NPR (i.e. nozzle pressure ratio) without base secondary flow and with secondary flow of 1.4% mass flux rate were obtained. The result shows that with the increase of ambient pressure, base pressure also increases. But under closed wake, the increase is slow; under open wake, the increase is rapid and even the pressure fluctuates.