A grating structure with the period of half of the laser wavelength is formed by focusing the atoms with a nearly resonant laser standing wave field. The motion of sodium atoms traversing an unsaturated potential are studied by the semi_classical approximation. The structures of gratings under various conditions were simulated using numerical method. By characterizing the grating with FWHM and contrast, the influence of some factors, such as the transverse temperature, the longitudinal velocity, and the free flight to the atom deposition are analysed. Our work provides a theoretical basis to the ongoing atom lithography experiment.