As one of the classical methods for image threshold segmentation,OTSU algorithm has been widely applied in image processing.An improved method based on Otsu algorithm for IR images is presented.Firstly,the average level is used as the initial threshold to segment images.Secondly,the recurrence formulas of four parameters are deduced.Finally,the threshold value T is calculated using the four formulas within the reduced gray level range.Experiment results show that the improved method can get better segmentation effect and reduce the calculation time significantly.