Abstract: In order to improve the accuracy of radiotherapy and reduce dose distribution errors caused by penumbra, a mathematical analysis model based on the maximum-radius arc intersection method was introduced. An optimization method for circular-arc end-face structures was proposed by taking the mean penumbra width within different field-size ranges as the optimization objective, and four end-face structures were designed. Under 6 MV X-ray irradiation, the penumbra widths of collimators equipped with four different end-face structures were measured under different field sizes using a three- dimensional blue water tank dose scanning system. The experimental results show that, compared with the traditional circular-arc end-face structure and the single planar end face, the optimized collimator end face based on the maximum-radius arc intersection method can achieve an ideal dose fall-off gradient in both small and large fields.