The accurate evaluation on the compensation range of part-compensation lens is beneficial to guide the design of part-compensation lens and realize fast testing on many aspheric surfaces. The prior evaluation methods based on asphericity have error judge occurred, thus reduce the compensation range of part-compensation lens negatively, further increase the testing time of aspheric surface. Therefore, an evaluation method based on slope asphericity that fits testing requirements is proposed to evaluate the compensation range of part-compensation lens. Mass simulations are processed to compare different evaluations and the effect of aspheric surface parameters on compensation range evaluation is analyzed. Additionally, the operation mechanism of part-compensation lens is analyzed from slope match of rays. The results indicate that accurate evaluation can be realized via using slope asphericity, thus is beneficial to design a part-compensation lens with the compensation range as large as possible.