The invention discloses a device and a method for detecting surface quality of steel plates and automatically positioning an illumination angle. The device comprises a bearing, stepping motors, a camera, a transmission shaft, a fixture, a line light source, a slide block, a base, a computer and a support frame consisting of an upright post, an upper transverse beam and a lower transverse beam, wherein two ends of the upper transverse beam and the lower transverse beam with the outer sides provided with the stepping motors are connected to a vertical slide block of the upright post; the camera is arranged on the inner side of the transverse beam; the transmission shaft fixed on the upper and lower transverse beams by virtue of the bearing is connected with the stepping motor; the transmission shaft is connected with the line light source; and the computer is connected with the stepping motor and the camera. When steel plates passes through the space between the two line light sources, a motion controller of the computer drives the stepping motors to rotate for a line light source illumination angle, line images of the steel plates are acquired by the camera, the line light source illumination angle value at the maximum line image gray value is automatically found, the most appropriate line light source illumination angle is judged, and the illumination angle is automatically positioned. According to the device disclosed by the invention, the maintenance workload can be reduced, the adjusting and positioning time is shortened, and the adjusting and positioning accuracy is improved.