There is a problem that the application and popularization of key secondary equipment such as a power system merging unit are affected by the lack of verification means for the measurement error of a merging unit tester. Thus a device for testing and calibrating such measurement error is developed. This is an important step in the promotion of intelligent substation construction. First, the disadvantages of the existing methods are analyzed. Then the overall idea of the measurement error test and calibration for the merging unit tester is proposed, and the design scheme is clarified. Secondly, based on a high-precision hardware acquisition loop and high-resolution ADC synchronous sampling technology, high-precision sampling of analog signal and high-precision output of the digital signal are realized to solve the problem of signal source. Finally, based on accurate timestamp technology and a sampling value correction algorithm, the high-precision and wide-range simulation of A/D error are realized as the reference error measured by the merging unit tester. Test and engineering verification results show that the acquisition output, A/D error simulation and anti-interference ability of the newly developed device meet the standard and application requirements, effectively solve the industry problem outlined above, and fill a gap, and improve test and calibration ability and efficiency. This work is supported by the National Natural Science Foundation of China (No. U1804252).