The invention aims at providing a surface cleanliness online monitoring system. The system comprises high-speed industrial cameras arranged on a front and a back of strip steel conveyed on a strip steel production line and used for shooting surface images of the strip steel conveyed on the strip steel production line; industrial camera lens arranged on the high-speed industrial cameras and used for adjusting shooting focal lengths of the high-speed industrial cameras; an image processing module connected with the high-speed industrial cameras and used for preprocessing the images shot by the high-speed industrial cameras and sending the preprocessed images to an industrial personal computer; and the industrial personal computer connected with the image processing module and used for identifying the cleanliness of the strip steel surface based on the images received from the image processing module. The automatic detection on the strip steel and like metal surfaces can be realized.