The third generation Beidou navigation satellite system（BeiDou-3） officially started full-scale global services on July 31， 2020. The Beidou satellites are designed to provide positioning， navigation and timing services， as well as certain specific functions， like local and global short message communication， satellite-based augmentation， precise point positioning and international search-and-rescue service. This paper introduces the functions of BeiDou-3 in terms of the design principles and new technologies that are integrated into the system. Some novel breakthroughs are particularly analyzed， including the satellite signal system， inter-satellite links， time and frequency standards， space-borne electronics， effective powers and overall automatic controls. Finally， the general design of satellites that specifically targets three kinds of orbits is reviewed.