The diffraction aberration of focusing atom beam of metastable helium in laser standing wave is studied by using quantum mechanics. A quasi classical model accordance with the existing model under the particular condition is built. The time dependent evolution operator method with fast computational speed and convergence is put forward to perform the calculations. Under given parameters of laser power and diameter etc, the simulation resnlts show that atom beam of metastable helium has been focused with a FWHM of 54 nm and contrast of 16∶1, the focal length and focal depth related to velocity and energy of atom beam. The diffraction effect contribute to the limited width of focus mostly.