The electromagnetic radiation of pantograph-catenary offline discharge has the characteristics of transient and broadband. We can use the D-dot sensor to measure its transient electric field in time domain. However, the direct integral operation of the differential signal seriously distorts the original signal. To solve this problem, a transient electric field time-domain waveform restoration system including a pulse electric field generating device and a measuring device is built in the laboratory firstly. Then, we propose a time-domain waveform restoration method for transient electric field including DC removal, numerical integration, elimination of trend items and low frequency compensation. Finally, the method is used to test the electric field time domain waveform of electromagnetic radiation of pantograph- catenary offline discharge under different voltages. Theoretical and experimental results show that the proposed method can accurately and stably restore the original time-domain waveform of the transient electric field radiated by off-line discharge of the pantograph. The main frequency components of the reduced signal and the measured differential signal are both at 7.5MHz, and the correlation coefficient between them is more than 93%.