Abstract: It is difficult to generate a bandpass prolate spheroidal wave function (PSWF) pulse signal in real-time, and the non-sinusoidal orthogonal modulation in time domain based on PSWF is sensitive to the synchronization timing error and hard to realize. For these issues, from the low-pass equivalent principle of bandpass signal, the low-pass equivalent form of the bandpass PSWF pulse signal is analyzed, and the equivalent orthogonal condition of the complex envelopes of two bandpass PSWF signals is derived, then, a demodulate method based on low-pass equivalent principle is proposed. The bit error rate(BER) performance is analyzed when the system is ideal or non-ideal synchronized. The mathematical deduction and simulation results both show that the demodulate method proposed is better when system is unideally synchronized. Compared with other demodulate methods, the method proposed at least improved the BER performance by 2dB, and the complexity of the system can be greatly reduced.