In optical lithography tools, thermal aberration of a projection lens, which is caused by lens heating, leads to degradation of imaging quality. In addition to in-line feedforward compensation technology , the thermal aberration can be reduced by optimizing projection lens design. Thermal aberration analysis of a projection lens benefits the optimization of projection lens design. In this paper, thermal aberration analysis methods using physical model and simplified model are compared. Physical model of lens heating provides accurate thermal aberration analysis, but it is unable to analyze the contribution of an element of the lens to thermal aberration which is significant for thermal optimization. Simplified model supports thermal analysis of an element of a lens. However, only the deformation of lens surface and the variance of refractive index are considered in the simplified model. The thermal aberration analysis, in this paper, shows not only the deformation of lens surface, the variance of refractive index but also the change of optical path should be considered in thermal aberration analysis. On the basis of the analysis, a strategy for optimizing projection lens design is proposed and used to optimize thermal behavior of a lithography projection lens. The RMS value of thermal aberration is reduced by 31.8% in steady state.