Since the number of available combinations is limited and the combined integer ambiguities are difficult to fix, an algorithm for direct calculation of BDS single-frequency integer ambiguity based on dual-frequency correlation constrains is proposed.On the premise that a combined integer ambiguity is fixed correctly, the single-frequency integer ambiguity can be calculated by the agency of the fixed combined integer ambiguity according to the correlation between B1 and B2 double differential equation, then margin of error of single-frequency integer ambiguity is determined through mean square error of the fixed combined integer ambiguity.Finally, the correct single-frequency integer ambiguity can be obtained through adaptive threshold OVT test method.The experiment results reveal that, when mean square error is less than 1/4 of wavelength, the algorithm can calculate B1 and B2 single-frequency integer ambiguity correctly with only one combined integer ambiguity, which means that the proposed algorithm is suitable for kinematic to kinematic high-precision relative positioning.