The invention discloses a method for measuring diffuse light reduced brightness coefficients of an auto-darkening welding filter. A light source, a sample, a composite diaphragm, an imaging lens, and a photoelectric detector are successively arranged on the same optical axis. The method comprises a step 1 of establishing a measuring device and setting a bright-state shade number; a step 2 of simultaneously acquiring the reduced brightness coefficient of an instrument with the set shade number and a total reduced brightness coefficient of the instrument including the sample; a step 3 of computing a self-generated brightness reduction coefficient l* of the sample with the shade number according to a following formula l* = l*g - l*a; a step 4 of increasing the shade number of the sample by using a shade number adjusting button of the sample, and computing the diffuse light reduced brightness coefficients of the sample with various shade numbers. The method has beneficial effects of eliminating a link replacing the diaphragm, simultaneously measuring optical signals of different spatial regions formed by different structural modes of the same diaphragm, simplifying measuring steps, and increasing measurement precision.