Abstract: Uncalibrated phase delay at the receiver end may depend on the frequency of the signal received, which leads to mixed double difference ambiguity between GPS and BDS and, lost integer character. In order to solve this problem, inter-system biases between GPS and BDS need to be estimated or corrected. Taking the biases of time system, coordinate system and frequency between GPS and BDS into consideration, a computing model of GPS-BDS inter-system bias is deduced, based on which, different experiments verify the stability of inter-system biases. Experimental results show that GPS-BDS inter-system biases of different brand receivers have stability under certain conditions. GPS-BDS inter-system biases are nott influenced by antenna type and antenna cable length significantly. GPS-BDS tight combining with inter-system biases corrected performs well under poor observing condition, and improves the mean time-to-ambiguity-fixing by 33%, and the success rate of ambiguity-fixing by 31%.