The invention discloses an oil filling system and method for a transmission. The oil filling system comprises an oil storage tank used for storing transmission oil, oil filling branches connected in parallel, and an oil pump used for conveying the transmission oil from the oil storage tank to the oil filling branches. Each oil filling branch comprises an oil filling control valve and a flow meter which communicate with each other in sequence. The oil filling branches communicate with oil filling ports of the transmission through oil filling guns correspondingly. The oil filling system further comprises a controller and oil level sensors. The oil level sensors are arranged on the oil filling guns. The controller is electrically connected with the oil level sensors, the flower meters and the oil filling control valves. The oil filling ports are further connected with an oil outlet branch communicating with the oil storage tank. By means of the oil filling system and method for the transmission, the reworked transmission does not need to be weighed, the oil filling process of the transmission can be completed on one work station, and accurate oil filling can be realized for the newly assembled and reworked transmission.