A grating structure with period of half of the laser wavelength generated by focusing Cr atoms with nearly resonant laser standing wave atom lens was simulated using a quantum-mechanical model. The influence of thermal atomic source on atom focusing, including the statistical distribution of the longitudinal velocity and the beam divergence, was discussed. The background and full-width at half-maximum (FWHM) of atomic density peaks with vz in Maxwell distribution and vx0 in Gaussian distribution increase significantly compared with ideal atoms. Collimating atoms with laser cooling is necessary to decrease the peak broadening.