The invention relates to a method for measuring grain diameter by utilizing binocular vision matching and particle picture gray scale difference, pertaining to the technical field of optical grain diameter measurement. According to the characteristic that different forward angle scattered lights have light intensity differences and by utilizing a digital particle image velocimetry (DPIV) imaging system provided with dual-cameras, the method obtains two particle pictures with gray scale difference, the flow fields of which are formed by single exposure; after the particle pictures are pre-treated by an image processing technique, the mating of a same particle corresponding to two particle image points on the two particle pictures is realized according to a binocular vision matching principle; the grain diameter of the particle is obtained by substituting the gray scale value summation of the two particle image points into a grain diameter solving formula, thus obtaining the particle size information of the whole flow field; the method has the reduced requirement to the light intensity and the stability of laser and the particle mating method is simple and convenient, thus reducing the influence of system errors, the digital graphic processing technique and laser diffraction on the measurement result of the particle size, overcoming the deficiencies and defects in the prior art and having substantial characteristics and prominent advancement.