In order to reduce the polarization sensitivity of a spectrometer, a method of designing a depolarizer in convergent optical path is proposed. In this method, the telescope and collimator of the system are regarded as auxiliary optical paths, so that the polarization state of the beam can be expressed by Stocks vectors; then the polarization characteristic of the element is represented by Mueller matrix, and the polarization sensitivity model of the system based on Mueller matrix is derived; finally, multiple regression analysis is used to improve the calculation speed and accuracy of Mueller matrix. Based on this method, an improved depolarizer placed in the convergent optical path is designed. The polarization sensitivity of the system is less than 1%, and the image point splitting distance is less than 8 μm. The results meet the design requirements and verify the effectiveness of the method for designing the depolarizer in the convergent optical path.