Abstract: To realize tidal correction for ocean bathymetry, an ocean multibeam non-tidal bathymetry method was applied based on a combined system of Global Navigation Satellite System (GNSS) and Inertial Navigation System (INS), and the basic principles and procedures of the non-tidal bathymetry are presented. Considering the form and filtering principle of the GNSS/INS system, differences in performance between PPP/INS loose combination and tight combination under valid or invalid state of GNSS were discussed. The outcome of the PPP/INS tight combination were used to correct the non-tidal bathymetry and compared with that of the traditional tidal correction method. It was verified that the accuracy of the non-tidal bathymetry in the PPP/INS tight combination mode could reach 0.14 m, and the influence of dynamic draft could be effectively eliminated, which improved the accuracy in open sea multibeam bathymetry and in bathymetric survey at the typical water depth section, performing obviously better than the predicted tide correction model.