A new approach to segment man-made target from infrared image is given based on threshold method. In order to reduce computation cost , a single method to select region of interest(ROI) is put forward so as to extract a piece of region ,which contains a target to be segmented. The histogram of ROI is smoothed by Bezier curve method to restrain noise. The ROI is segmented based on threshold method and a new criterion proposed. Final segmentation result is obtained by combining the segmentation result above with edge information of target. Experimental results show that a target can be segmented effectively from complex background in infrared image by the new approach.