Abstract: In order to solve the problem that the merge unit(MU) error calibration needs to outage substation, a self-monitoring method for merge unit errors based on the dual-reference source calibration method is proposed. In this paper, the reference signal is injected into MU acquisition circuit, and after sampling by A/D, the standard signal features are extracted by all-phase spectrum analysis method, and the errors of MU acquisition circuit are judged according to the amplitude-frequency characteristics. The main and secondary reference signals are designed to ensure the technical reliability and solve the problem that the performance attenuation of reference signals during operation affects the self-monitoring function. The AD9834 chip is used as the core component of the reference signal source, the ARM and FPGA are used to separate and extract signal features. A test platform is set up to test the self-monitoring function of the MU. The results show that the voltage accuracy class of the self-monitoring MU is 0.03, the current accuracy class is 0.05. Self-monitoring MU can detect the error caused by the change of the resistance reactance parameter of the acquisition circuit. In the test environment simulating the actual substation project, MU can detect the voltage amplitude error of ±0.05%, the voltage phase error of ±2', and the current amplitude error of ±0.05% and the current phase error of ±2'.