Errors of network parameters can severely affect the performance of state estimation and degrade the usefulness of other advanced applications in an energy management system（EMS）. The work on processing error of parameter estimation is briefly reviewed in the paper firstly. The method for parameters estimation based on the sensitivity relationship between the measurement residuals and parameter errors are discussed in detail. Following the framework of the two-step parameter estimation method， the approximate models of the bias vector are analyzed in detail. To use it in complex cases， this paper improves the calculation of bias models of the first step and extends the parameters estimation method in the second step to include the estimation of any impedance parameter. Test results of the improved method are presented and show that the method can have good application prospect.