There is a large workload involved in on-site commissioning of reconstruction and expansion of a smart substation. The numbers of pieces of primary and secondary equipment have increased significantly, as has the time involved. This paper proposes a mirroring debugging technology for smart substation reconstruction and expansion, based on the division of reconstruction and expansion domains and the consistency of sub-CRCs before and after reconstruction. It can automatically identify the test boundary of the reconstruction and expansion based on the division of the reconstruction domain and a consistency comparison of the sub-CRC before and after the reconstruction. It verifies the correctness of the virtual circuit connection between devices through automatic identification of test boundaries and virtual circuit mirroring simulation transmission. Based on an MMS model, the relay protection equipment of an intelligent substation realizes closed-loop automatic test during technical transformation and replacement of reconstruction equipment. It guarantees the correctness of the message model parameters before and after the reconstruction and expansion of the operating interval by comparing the heartbeat messages before and after the reconstruction. This paper also designs a mirroring debugging system for the inspection, modification and expansion of smart substations, and takes a 220 kV substation reconstruction and expansion test as an example to verify the effectiveness of the proposed technology. This work is supported by the Science and Technology Project of China Southern Power Grid Corporation (No. GXKJXM20190610).