Quantitative evaluation of signal quality of navigation satellites in orbit is one of the difficulties in satellite navigation signal quality monitoring research,so a quantitative evaluation method based on theoretic ranging bias is proposed.Based on Vernier sampling principle and a large diameter parabolic antenna is used for in-orbit satellite signal collection,and a clear waveform of code epoch is achieved after data processing.Based on the clear waveform,it’s starting point is estimated,and leading signal and delay signal are produced,then the E-L discriminator curve is got,finally zero crossing bias is achieved,which is the ranging bias of the signal.A large diameter parabolic antenna is used for COMPASS satellite tracking,and signal collection and data processing is made,then theoretic ranging bias of I channel atB1band of several COMPASS satellites are obtained,so the estimation method is verified.