Long wave infrared thermal imager based on TDI (Time Delay Integration) mechanism is widely used in military, aviation and aerospace fields. Through optical scanning, large imaging field of view and high spatial resolution can be achieved simultaneously. The detector of the long wave infrared thermal imager based on TDI mechanism adopts TDI working mode. Due to the problem of matching accuracy between TDI and optical scanning, when imaging a scene, the edge of the scene often has uneven sawtooth, which affects target extraction and recognition and reduces the image appearance. In this paper, the phenomenon of aliasing in the scene image caused by the mismatch between the scanning field of view and the optical field of view is analyzed, and a method to eliminate sawtooth by converting the spatial physical interval of the pixel distribution on the detector into a time delay is proposed. The method of time to space can effectively eliminate the sawtooth of the scene image, restore the details of the small target edge, greatly improve the image appearance, and the method has been widely used in IRST systems, infrared photoelectric systems and other equipment.