Volume holographic correlator has advantages for the fast optical parallel recognition. However, because of the inter page crosstalk, the sidelobes occur seriously, which reduces the parallel-processing capacity of the correlator, resulting in the low accuracy of the correlator. To suppress the sidelobes, the traditional correlation formula is analyzed to discuss the origin of sidelobes. It is proposed to modify the formula with the autocorrelation function of a random function, which can suppress the sidelobes in the horizontal direction as well as in the vertical direction, with no dependence on the thickness of the hologram. A diffuser is proposed to be placed in the object path to modulate the object beam with speckle. The theoretical analysis, numerical simulation and experimental results are presented and it is shown that this method can suppress the sidelobes, sharpen the correlation peaks and improve the correlation veracity.