Abstract: Aiming at the problem of rank-defect and ill-conditioned ambiguity resolution in the process of single frequency single epoch combined RTK positioning, a new method for ambiguity decorrelation is proposed, in which pseudorange observations is introduced as auxiliary solution.First, we adopt the empirical decentralization method to assign weights to the pseudorange and carrier phase observations, and obtain the floating resolution of ambiguities by weighted least squares. Then the ill-conditioned ambiguity resolution can be eliminated by the descending order of variance-covariance matrix of the floating resolution of ambiguitiesand Cholesky triangular decomposition. Finally, we use the modified floating solution toiterate the search of the integer solution of ambiguity. The experimental results show that the down-correlation algorithm can not only improve the positioning accuracy, but also can reduce the correlativity of the ambiguity, which is suitable for RTK positioning.