Using the Gaussian correlation model of random surfaces, we conduct the theoretical derivation of the image-plane intensity produced by a weak scatterer in the 4f optical filtering system.Then the method is proposed for characterizing the square-root-mean deviation roughness and lateral correlation length.This method uses the derived theoretical expressions to fit the experimentally measured curve of image speckle intensity versus the radius of filtering aperture,and the two surface parameters can be extracted simultaneously.In the experiment,random surface samples are specially made and their surface parameters are measured. The results conforms with those obtained by atomic force microscopy,which shows that the method of this paper is of good accuracy.