Three-phase voltage source converter (VSC) systems are widely used in the industry. The open-switch faults of VSC systems are not noticeable, but they can result in a major disaster. Aimed at this problem, an open-switch fault diagnosis method using tracking differentiator is proposed. It is found that both single and multiple open-switch faults can cause significant changes in differential current, which can be utilized to diagnose the faults. The tracking differentiator is used to estimate the differential currents without circuit characteristics or additional hardware. With the help of it, both single and multiple open-switch faults can be diagnosed by obvious features, which avoids the comparison of multiple parameters. Furthermore, the proposed method exhibits excellent anti-interference performance as well as high robustness under a variety of conditions. The efficiency of the method was demonstrated by the results of the experiments.