The sliding spectral (SS) method of processing radio occultation data in multipath zone is introduced in detail and compared with back propagation (BP) method through processing simulated data and COSMIC soundings. The modeled signals are inverted with the use of two methods: BP method and SS method. Both methods can solve the problem of calculating bending angle profiles in multipath regions. Better agreement with Abel integral is indicated by the BP method. The BP method is almost free from noise, while the SS method will be affected severely by noise. About 4500 COSMIC radio occultation soundings are retrieved by the BP method and the SS method, and the statistical comparison of retrieved refractivity with that from ECMWF analyses shows that the SS method has better penetration ability than the BP method in the lower moist troposphere. The mean of fractional difference in refractivity retrieved by the BP method is generally smaller than that by the SS method.