An automatic paint spraying machine for a bridge guardrail relates to paint spraying equipment for the bridge guardrail. A grooved wheel (22) is fixedly connected to the middle of a shaft (21); a vertical bevel gear (13) is arranged on the shaft on one side of the grooved wheel; a motor (15) is arranged on the inner wall of a power supply box body below the vertical bevel gear; a horizontal bevel gear (14) on the power output shaft of the motor is meshed with the vertical bevel gear; the motor is connected with a storage battery (17) and a control button (2) respectively; the storage battery is arranged on the inner wall of the power supply box body; the control button is arranged on the outer wall of the power supply box body; a container (20) and a high-pressure pump (19) are arranged on the inner wall of the power supply box body on one side of the grooved wheel; a paint spraying work box body (11) adopts an II-shaped structure; a plurality of spray nozzles (16) are uniformly arranged on the paint delivery pipe (8) of the II-shaped structure; and the paint delivery pipe is connected with the high-pressure pump. The automatic paint spraying machine has the advantages that the paint spraying operation on the guardrail is mechanized; and the operation strength of part of people as well as the occupied high-speed rail running time is effectively reduced.