The formula of the generalized ambiguity function of passive imaging system using Phase Shift Keying(PSK) modulating signals is derived in detail. Considering the characteristics of signals, the concept of average ambiguity function is introduced to remove the effect of particular sequence of symbols on the ambiguity function. Then the influence of the waveform parameters of the PSK modulating signals on spatial resolution is investigated by numerical simulations. The results indicate that the influence of the length of the symbol sequence and the roll-off factor is very slight.