Abstract: In order to study the effect of illuminating beam on output correlation spots,for a given target image,the relation among the output correlation peak value,the parameters of Gaussian beam and the location of the image on the input plane was simulated with the joint transform correlator illuminated by Gaussian beam.It was found that the output correlation peak value decreased with the decrease of the waist radius of Gaussian beam and the increase of the interval between target image on input plane and the center of Gaussian beam.Simulation results show that the ratio of amplitude of the center of Gaussian beam before and after beam expansion should be 5.7 if the waist radius of 1mm is expanded to 32mm,where the output correlation peak value at the edge of the facula is 90% of that at the center,and that in order to output high correlation peak value when images locate any place on the input plane,the waist radius of Gaussian beam must be expanded big enough.