Usually， electromagnetic transformer is still in use during the digital substation construction. The electromagnetic transformer accessing solution during digital substations modernization is introduced. The causes of errors generation are analyzed when the electronic and electromagnetic transformers simultaneously access for bus differential protection. Through the delaying parameter setting of sampling data， the method to eliminate errors is put forward. Data synchronization between two kinds of transformer is discussed. Based on the adjustment of receiving and sending sequential control logic， the problem of synchronized transmission for the merging unit sampling data is solved. A solution for the practical case of Sanxiang digital substation modernization is proposed.