In this article, we investigate the problems existing in the lithography on the deep multi-stepped surface. Different photoresist thicknesses above and under the step are measured in experiment. The relationship between step height and photoresist thickness is discussed and numerically described. Based on the description of Beer model about the light absorption coefficient, the curves of different light transmittances at different times are analysed. Reasons why the light transmittance changes with time are explained, and the light absorption coefficient is believed to be related to photoresist thickness. On this basis, the lithography process is optimized. The patterns with narrow line-width under the deep step on the wafer are obtained.