Motor protector is a complex product and has powerful protection function. It has high accuracy and wide range requirements for current regulating. It was constructed by combining coarse and fine voltage regulators to form wide range and high accuracy alternating constant-current source. The software was programmed by using LabVIEW software to realize data acquisition, processing, real display and real control. The auto testing device based on virtual instrument was constructed with data acquisition card, control system and drive circuit. Practical application shows that the system has solved the shortcomings of traditional test method of low accuracy and efficiency. It has a good promotion prospects.