The invention discloses an automatic filling machine for a battery and a battery filling method. The filling machine comprises a rack, a front weighing device arranged on the rack, a filling conveying line, an evacuation filling device, a discharging device, a rear weighing device and a control module. The filling machine also comprises a carrier backflow mechanism which comprises a carrier conveying line parallel to the filling conveying line and carrier transfer devices arranged at two ends of the filling conveying line and the carrier conveying line; and the evacuation filling device comprises a filling nozzle, a driving part for driving the filling nozzle to move, a filling cup communicated with the filling nozzle, a filling buffer tank communicated with the filling cup and a sealing cylinder for sealing the filling nozzle and the battery in the sealing cylinder. In the invention, automatic operations of battery conveying, filling, evacuation and discharging are ensured, the machine design and process layout are compact, the vacuum and high-pressure circulating alternate filling method is adopted, and the full absorption and filling precision of electrolyte of the battery are ensured.