Abstract: Objectives: The LuTan-1 (LT-1) is the first formation-flying mission of China, enabling interferometry synthetic aperture radar (InSAR) interferometry in the L-band. High-precision intersatellite baseline is crucial for InSAR processing and application. Methods: We investigate the dynamic precise baseline solution method and the inter-satellite double-difference ambiguity fixing method of low-orbit formation satellite, and analyze the contribution of BDS-3 (B1C, B2a) and GPS (L1, L2) observation of LT-1A/B in precise baseline determination. Results: The results show that compared with GPS, the residuals of B1C and B2a observations of BDS-3 are significantly smaller, suggesting a higher signal accuracy. The baseline overlap accuracies of GPS-based, BDS-based, and GPS/BDS combined solutions are 0.8 mm, 0.6 mm, and 0.5 mm, respectively. The BDS-based baseline accuracy is 25% higher than that of GPS, and the GPS/BDS combined solution can further improve the baseline accuracy by 37.5% and 16.7% relative to the GPS- and BDS-baselines. The baseline difference between the GPS-based and BDS-based baseline solutions is 1 mm in 3-dimensions directions. Conclusions: The analysis suggests that the single BDS-3 system can achieve precise baseline determination for low-orbit formation satellites at the 1mm level and combined GPS/BDS solution will further improve the accuracy than single-system solution.