In this paper, the principle of power spectrum subtraction for images with multiple grey levels through the phase modulationis discussed. And, based on this principle, a new experimental solution of the optical joint transform correlation for multiple objects with multiple grey levels is presented. In this experimental solution, the power spectrum is recorded two times and followed by a subtraction of them. During the second recording, a phase plate is used to produce a half-wave delay of the reference image. The subtracted power spectrum is used for reverted Fourier transformation, which results in enhanced correlation peaks and eliminates correlations between the objects and between the background noises. This method is specially suitable to the recognization of multiple objects with multiple grey levels and is easy to be implemented.