Abstract: Objective: To diagnose and locate the fault range in eightwire turnout control circuit, a fault diagnosis and analysis method for the above circuit of AC switch machine is proposed. Method: The power, voltage, control circuit key relay status, and indication voltage acquisition method of the eightwire turnout are introduced. When carrying out fault analysis for the turnout indication circuit, the nonactivated status of 1DQJ (1st starting relay) leads to judgment of faults in indication circuit. Based on the collected indication voltage of the branch panel, the fault range is determined，and the detailed judgment process for turnout in the located and reverse positions is explained. In fault analysis on turnout starting circuit, the diagnosis methods are introduced corresponding to three parts: failure to actuate after turnout operation, faults during the actuation process, and faults after the actuation is completed. Result & Conclusion: Results of multiple tests show that the proposed fault analysis method can accurately diagnose the fault causes and locate the fault range for the eightwire turnouts.