We design an automatic test system based on a digital supermode distributed Bragg reflector laser, electro-optical modulation polarization controller, InGaAs photoelectric detector, high-speed optical switch, and a combination of high-precision control-detection algorithm and wavelength-division multiplexing technology. The proposed system is designed to automatically test performance-related indicators, such as wavelength and polarization dependent losses of passive optical components. To realize excellent control of power stability, the light source module of this system uses the OPA569AIDWP chip as the control core. Multi-grade amplification range control is used for power detection module. Further, shielding noise reduction technology and software algorithms are used to significantly suppress system noise and improve the power meter sensitivity and dynamic range. The test process of the proposed system is simple, highly accuracy, and stable, with a test deviation of less than 0.05 dB, which improves the automatic detection capability of the passive optical device production line and reduces the requirements of manual operation level for using complex instruments. The equipment practicality of the proposed system is good and it shows its broad market prospects.