Both the theoretical analysis and computing simulation were done for the photon correlation spectroscopy (PCS) of an actual particle system,and the results showed that the relative error of the reversed particle size could be up to 100% if the mixed PCS was reversed only by the self-beating or heterodyne arithmetic.So we presented a method of reversing the particle size from a mixed PCS.A simulated experimental arrangement which can give rise to the mixed PCS was built.Two particle systems,composed of standard polystyrene latex spheres with nominal diameters of 60nm and 200nm,were examined with the arrangement.The experiment result shows that compared with their nominal values,the relative errors of the reversed particle sizes are all within 5% when the mixed PCS is reversed by the proposed method.