Abstract: With the continuous development of China BeiDou Navigation Satellite System(BDS), it has entered the third stage of construction, and its positioning accuracy has always been a hot topic of global concern. Based on the multi-mode GNSS data of the IGS continuous tracking station on the 227th day of 2018, this paper dynamically processes the short baseline, analyzes the relative positioning accuracy of the BDS-3 system, and analyzes the relative positioning accuracy of the BDS-2. The results show that the relative positioning accuracy of the short-baseline of BDS-3 is better than 3 cm in the E direction, the N direction is better than 3.5 cm, and the U direction is better than 10 cm. The accuracy in the three directions is slightly higher than the short positioning accuracy of the BDS-2 short baseline.