Abstract: We analyze the reason that zero-differenced ambiguity cann’t be fixed in a standard mode.We proposed the FBI(fractional bias isolation) mode and method for zero-differenced interger ambiguity resolution.We found that with 30 min observations,the accuracy of mm-level in horizontal and cm-level in elevation direction can be achieved,which has respectively enhanced one order of magnitude and has improved approximately 50%-60% in horizontal and elevation direction compared to float solution.In the meantime,the accuracy of ZPD(zenith path delay) estimation has been also ameliorated about 20%-30%.