The invention relates to a method for measuring particle velocity using grey scale autocorrelation of particle image, belonging to the technical field of optical measurement of velocity. The method comprises obtaining particle image with grey scale difference formed by double exposure of flow field using an imaging system of a particle image velocity field meter, pre-treating with digital image processing technique, pairing particle image points corresponding to double exposure of the same particle in a divided sub-region according to the total grey scale of the obtained particle image points and the internal correlation with different widths of laser optical pulse, and obtaining the velocity vector of the particle according to the position information of two particle image points and the difference of the total grey scale, so as to obtain the velocity information of the total flow field. The invention reduces the performance and control requirements to camera, reduces the influence of Gaussian distribution of laser optical intensity on the results, so as to improve the timeliness of velocity measurement, and solve the direction ambiguity problem. The invention is characterized in substantive distinguishing features and marked improvement by overcoming the shortcomings in the prior art.