In the lithography process of thin film transistor, the lithography plane position of photoresist is the key factor that determine the quality of lithography pattern. In order to improve the quality of lithography pattern under the minimum resolution of lithography machine, the reflection characteristic of the light in the photoresist is studied in this paper, based on reducing the intensity of the reflected light in the photoresist on the non-lithography region and increasing the intensity of the photoresist at the bottom on the lithography region, the computational formula for the lithography plane position adjustment of the photoresist is deduced under the oblique incidence. The adjustment amount is calculated by the formula and the lithography plane is adjusted by the adjustment. The results show that for the projection lithography machine with the minimum resolution of 3.0 μm, and for the product with the line space of 2.2 μm, after adjusting the lithography plane of photoresist with this adjustment，the slope angle of the lithography pattern is increased by 13.3%，and the uniformity of the DICD (development inspection critical dimension) is improved by 14.7%，the photoresist remain of the lithography pattern is resolved.