Numerous studies show that power semiconductors are significant contributors to the overall failure rate of PWM voltage source rectifier (VSR) systems. Aimed at this problem, this paper presents an improved diagnosis system for opencircuit faults of a three-phase PWM VSR based on AC current distortion characteristics, which consists of a diagnosis module, a register module and a decision module. The diagnostic module employs a hysteresis comparator to judge the change trend of the currents, and thereby diagnoses the single OC faults. The register module is employed to achieve multi-switch OC fault diagnosis, and the decision module is used to output the final result. The proposed method extends the previous work to multi-switch OC fault conditions as well as improves the diagnosis accuracy. The experimental results show that the proposed method can correctly achieve fault diagnosis, including 6 kinds of single switch faults and 15 kinds of different double switches faults. Additionally, the proposed method also shows superior antiinterference performance and high robustness on abrupt load transient conditions, different power factor conditions, unbalanced and/or distorted grid voltage conditions and different AC filter structure conditions. Furthermore, the average diagnostic time of this method is only 3.68 ms.