A new method for evaluation of ambiguity-to-signal ratio in spaceborne bistatic SAR was proposed in terms of bistatic geometry. Based on that, the ASR of the new system was analyzed in different cases. Simulation results show that due to the limited size of antenna onboard small satellite, the ASR of bistatic SAR is much higher than that of monostatic spaceborne SAR, though their changing trends are similar. Compared with the ASR in azimuth, the ASR in range is closer to that of the monostatic case because the size in height dimension of small antenna is closer to that of the big antenna than the length dimension is.