Abstract: Satellite-to-satellite tracking is one of the most promising and valuable methods for gravity field determination. The high accuracy inter-satellite ranging system is the major payload of gravity exploring satellite using satellite to satellite tracking in low-low model. K Band Ranging system （KBR） on the Gravity Recovery and Climate Experiment （GRACE） satellite is a micron-level ranging system. The geopotential is determined by processing precise range or range rate observed between the two satellites. GRACE has ushered in a new era for satellite measurement of the Earth system. The tremendous advances made by GRACE have led to an interest in launching a follow-on mission with even more ambitious scientific goals. A higher accuracy laser interferometric ranging system is proposed for GRACE follow-on mission. A strawman flight design, an error budget, and a demonstration laboratory breadboard are developed in foreign country. Based on the study of KBR and laser ranging system＇s measurement principle, the fundamental structure of KBR was presented in this paper. The key technology of these two ranging system and the current research level of our country are analyzed in detail. At the same time, some suggestions are proposed for the future inter-satellite ranging project of China.