Abstract:There is a obvious north-south striping noise in the inversion results of GRACE unconstrained monthly time-varying gravity field spherical harmonic coefficient model. We provide an improved PnMl method that fits the high degree coefficients with hierarchical polynomial in order to better suppress the north-south striping noise. Further, we analyze the performance of two methods in the frequency, time and spatial domains. After applying the improved method, the residual striping noise between 30°S~30°N is further reduced, the anomaly amplitude uplifts beyond 50 degrees are suppressed, and the abnormal peaks in mass change time series are diminished. Besides, the SNR index and three-cornered hat method are used to evaluate the performance of improved method. The results show after processing by this method, the gravity field models provided by Center for Space Research(CSR), GeoForschungs Zentrum Potsdam(GFZ), and Jet Propulsion Laboratory(JPL) achieved an optimal SNR of 1.77, 1.35, and 1.39, respectively, while reducing their uncertainty by 20.50 mm, 36.40 mm, and 19.61 mm compared with P4M6 method.