In order to reduce the non-uniformity error caused by full frame average infrared calibration, a segmented calibration method of infrared systems suitable for external field conditions is mainly explored in this paper. In this method, the gain coefficient matrix is estimated first, then the estimation matrix is used to divide the calibration region, and finally the gain coefficient calibration matrix is obtained by using the least square method. The accuracy of the proposed method is verified by carrying out calibration experiment of large aperture ground infrared measurement system. The results show that, compared with the traditional method, the average relative error is reduced by 13.07% and the average error deviation is reduced by 0.2877 when the calibration equation obtained in this paper is used to invert the target radiation brightness. This method has higher calibration accuracy for the whole frame, and the accuracy of the object characteristic inversion is significantly improved. This method can provide useful reference for the practical engineering application of infrared system calibration.