Steadystate methods and transient methods can't give the accurate results when the coupling impedance and threephase unbalance load exist in power system. In this paper, the improved steadystate method is proposed to obtain the system harmonic impedance. Firstly, the phasemode transformation matrix is calculated which can decouple the three measurements into a set of single variables and simplify the calculation process; Secondly, singlephase model is established by MATLAB/Simulink to compare steadystate methods with transient methods; Thirdly, threephase model is established which exists the coupling impedance and threephase unbalance load. The steadystate methods and the improved steadystate methods is applied to calculate the the system harmonic impedance. The results show that the steadystate methods is more accurate than the transient methods and have less interference; The steadystate methods is improved by the phasemode transformation and the improved methods is selected to obtain the system harmonic impedance which improves the accuracy and simplifies the process of calculation.