为了设计低投射比的超短焦投影物镜，本文采用自由曲面和折反式的光路结构设计了一种具有低投射比的超短焦投影物镜系统。该物镜由一个旋转对称的折射透镜组和一个自由曲面反射镜组成。采用11938 mm的数字微镜器件(DMD)作为空间光调制器产生图像源。采用法线加权迭代优化的方法计算自由曲面。最后，分析了系统的性能。仿真结果表明：超短焦投影物镜可在580 mm的投影距离处实现3 048 mm尺寸的大屏幕投影，系统的投射比低至0.19，系统的最大畸变小于0.72%。能够满足低投射比超短焦投影物镜的设计要求。该投影系统具有低投射比、低畸变、投影效果好等优点，可为超短焦投影系统的进一步发展提供有益参考。

In order to design an ultra-short-focus objective lens with a low throw ratio, a system of ultra-short-focus, low-throw-ratio objective lenses is designed using a freeform surface and refractive-reflective optical structure. The objective lenses consist of a rotationally symmetrical refractive lens group and a freeform surface mirror. A 11938 mm Digital Micromirror Device (DMD) is used as a spatial light modulator to generate the image source. The normal-weighted iterative optimization method is used to calculate the freeform surface. Finally, the performance of the system is analyzed. The simulation results show that the ultra-short-focus projection objective lens can achieve a 3 048 mm screen projection at a distance of 580 mm. The throw ratio of the system is 0.19, which is extraordinarily low, and the maximum distortion of the system is less than 0.72%. This can meet the design requirements of low throw ratio ultra-short-focus projection objective lenses. This projection system is advantageous for its low throw ratio, low distortion and good imaging quality, which can provide a useful reference for the further development of ultra-short-focus projection systems.