With the increase in the speed of locomotives, the offline discharge between the pantograph and catenary (PAC) occurs occasionally, which generates the electromagnetic disturbance. The main purpose of this article is to study the electrical behaviors of the offline discharge between the contact wire (CW) with a high current and the pantograph under their relatively high-speed motion, including the characteristics of the discharge voltage, current, and electromagnetic disturbance. By analyzing the experimental data measured by advanced high-power PAC equipment, two discharge forms were found macroscopically. Under different supply currents and speeds, the time and frequency domain characteristics of the discharge voltage, current, and surrounding electric field (EF) were obtained. The main factors causing electromagnetic disturbance were discussed. Then, from the perspective of the migration-diffusion motion of the particle, the main reasons for the pulsation of the discharge current during the PAC offline were explained. Depending on the frequency distribution of the measured results, a 3-D model was built for the research on the PAC offline electromagnetic disturbance. Also, the variation laws of the electromagnetic field in the near and far-field were got. The research results of this article provide the references for the electromagnetic protection of the railway wireless equipment.