Abstract: Topography and gravity anomalies are two of the most fundamental observations of a planetary surface. Their magnitude and pattern reflect both the shallow lithospheric flexural isostasy and the deep mantle dynamics. Mantle dynamics primarily influence medium- to long-wavelength topography and gravity anomalies, with mantle viscosity being one of the most critical parameters controlling the dynamic behavior of a planet. The joint analysis of gravity and topography data has become an important method for studying the internal viscosity structure of planets. On Earth, the analysis of seismic waves can provide detailed internal structures, which are used as driving forces for mantle convection in geodynamic simulations. These simulations have successfully explained geoid anomalies and provide important constraints on mantle viscosity. The topography and gravity anomalies from mantle convection models can be combined with other data, such as plate motion, post-glacial rebound, and mineral physics experiments, to jointly constrain the mantle's viscosity structure. The method of using topography and gravity anomalies for dynamic modeling to invert mantle viscosity structure can also be applied to Venus. The main difference from Earth is that Venus lacks seismic data on internal density structures. Instead, numerical simulations of thermal evolution can be used to generate the present-day mantle structure, thereby constraining Venus's internal viscosity structure. The primary difference in mantle viscosity structure between Earth and Venus is the absence of a pronounced viscosity jump between the upper and lower mantle on Venus and the likely existence of an asthenosphere on Earth. The absence of an asthenosphere in Venus's mantle is the reason why Venus lacks plate tectonics. With the improvement in the precision of observational data and the advancement of research methods, planetary topography and gravity have become increasingly significant in the study of mantle viscosity.