A grating nanostructure generated by focusing Cr atoms with nearly resonant laser standing wave (SW) atom lens was numerically simulated using the quantum-mechanical model. To improve the imaging quality of atom lens, theoretical analysis was performed to describe the aberration of atom lens, including diffractive aberration, spherical aberration, chromatic aberration, the aberration caused by atom beam divergence angle, by magnetic sublevel structure and by isotopes. It was shown that quantum approach is more precise to describe atom focusing compared with particle-optics approach. The diffraction of atoms in laser standing wave (SW) field was explained with quantum approach. It was also shown in simulation results that aberration originates mainly from beam divergence, which is much larger than the diffractive aberration, the spherical aberration and the chromatic aberration. The aberration caused by magnetic sublevel structure and isotopes are small enough to be neglected. It is a useful method of collimating atomic beams with laser cooling to decrease the aberration caused by beam divergence. Chromatic aberration can be decreased by narrowing the statistical distribution of atomic velocity Vz.