This paper briefly describes the application and principles of the critical impedance method (CIM) in the main harmonic source detection, focusing on the analysis of the reasons for the CIM errors in both ends of the source network, and the effect of the error on the main harmonic source detection. Single rank Broyden method is proposed to correct the critical impedance, which has the advantages of smaller amount of computing than the Newton method, less dependence on the initial value, etc. MATLAB simulation of two examples show that using this method to correct the critical impedance can accurately identify the main harmonic source, greatly improve the accuracy of critical impedance method in the harmonic source detection, and enhance the practical value of the critical impedance in the main harmonic source identification.