A method for detecting degree of particulate contamination on a flat panel mainly includes the following steps: illuminating a to-be-detected flat panel (40) by using a light source module (10), to form an illumination field; adjusting a half width of the illumination field; adjusting a luminous intensity at a center of the illumination field and a luminous intensity at an edge of the half width of the illumination field; adjusting a light intensity and a position of the light source, as well as a position of a detector (20); and acquiring signals from foreign objects on the flat panel by using the detector (20). This method greatly alleviates particle mirror crosstalk and crosstalk of patterns on the lower surface of the flat panel, and improves the SNR, thus enhancing the accuracy in detection of foreign objects on the flat panel.