A transient sampling preposing unit for an electronic mutual inductor and merging unit transient test is composed of a precise current converter, a signal processing circuit, a synchronous AD converter, a constant-temperature crystal oscillator, and an FPGA field programmable gate array. The signal processing circuit of the unit comprises a signal conditioning module for standard source transient current signals converted by the precise current converter, a signal conditioning module for transient small voltage signals of a standard source, and a signal conditioning module for small voltage signals of an electronic mutual inductor sample. The three paths of signals after signal conditioning are led into the synchronous AD converter. The synchronous AD converter is connected with the FPGA field programmable gate array, and output is conducted through an optical fiber serial transmitter. The unit of the utility model is characterized by high sampling speed, high testing precision, good saturation feature, good real-time testing property, excellent interference immunity, high security, and the like. The unit of the utility model can accurately test the digital quantity transient precision of a merging unit based on the discrete data transmission, and thus provides a testing basis for wide application of intelligent transformer substations.