Numerical simulation on optical cross section (OCS) of complex targets is shown.An adaptive Z buffer method is discussed.Optical hidden facets are processed precisely and subtracted from integral OCS.Optical scattering characteristics described by bidirectional reflectance distribution function (BRDF) are used in numerical simulation.For surface pleats of space targets with grass-like optical scattering characteristics, normal vector model of pleat and scattering model are analyzed.An analysis for OCS experimental evaluation is given.OCS for a downsizing space target with different azimuth and zenith angle attitudes are achieved.Simulation parameters for downsizing target are calibrated with experimental results.Simulation images and trendlines are coherent with experiment results roughly, which indicates that the method describes OCS at a certain extent.