Conventional parameter identifying methods always encounter numerical instability， divergence and residual contamination. This paper proposes a new method to overcome these problems. First， the relative residual is worked out with measurement from the wide-area measurement system（WAMS） to preliminarily judge the existence of parameter error. Then the new method makes full use of the voltage and current phasors gathered by phasor measurement units（PMUs） to establish a direct relationship among the variables at both ends of the branch， with which， the branches with parameter errors can be identified and their parameters estimated with the optimization method as well. Simulation results on the IEEE 39-bus system show that all the parameters of transmission lines and transformers in a power grid can be monitored as long as half of the buses are equipped with PMU.