The invention discloses a mobile AGV battery replacing system and a battery replacement method thereof. Since an AGV needs to come back to a charging pile for charging or replacing a battery when thebattery capacity is not enough for completing remaining work, the efficiency is low. The mobile AGV battery replacing system includes a battery, a mobile platform and a battery base station. The polesof the battery are arranged at the same end of the battery. An attracting block is arranged at the other end of the battery. The mobile platform includes a mobile base frame, an advancing driving assembly, a first rotating shaft, a first chain wheel, a battery temporary storage pipe, a first shifting motor and an electric push rod. The battery base station includes a base, a second rotating shaft, a second chain wheel, a charging assembly, a charging guide rail and a second shifting motor. The charging assembly comprises a storage tube, a charging block and an electric brush strip. Replacement of the battery can be carried out for the running AGV, and waste of time and energy in the process that the AGV goes to and returns from the charging station is avoided. Replaced batteries with lowelectric quantity can be automatically charged.