Merging units have become a kind of important equipments for information collection in smart substations, however, the testing technology on merging unit has lagged behind. Currently, the test of merging unit often requires the use of a standard transformer, a precise oscilloscope and other instruments to complete various test items, and the operation is complicated. On the other hand, in smart substations, without use of the merging units qualified after testing, incorrect action of relay protection may arise, resulting in serious consequences. Therefore, it’s urgent and necessary to raise the level of the testing techniques on merging unit to satisfy the needs in actual production. A method based on ARM for the implement of analog-input merging unit tester is presented in this paper. The tester integrates accuracy testing, communication interface testing, synchronization testing and other performance testing, and can be used to test the merging units with traditional analog signal or small analog signal input. It could simplify the testing process of merging unit greatly.