To address the issue of image blur due to high-speed relative motion (approaching or moving away) between an infrared imager and a detected target, the principle of blurring caused by information mixing between image pixels as a result of high-speed relative motion is analyzed and derived on the basis of the target’s radiation characteristics and optical Fourier analysis. Firstly, with omnidirectional variable-speed linear motion blur as the main focus, the expansion or contraction of pixel values is added. Secondly, according to the response modeling of traditional infrared imaging systems, a nonlinear mapping relationship is established between the change in pixel grayscale values during exposure time and the imager speed. And A mathematical model of the high-speed motion expansion effect of the imager is developed and its point expansion function is given, which in turn plots the modulation function (MTF) curve. Further, using the theoretical MTF to design filters, the degradation of the test infrared square target image is performed to achieve the simulation of the blurring effect of the infrared image expansion effect. Finally, taking a ship driving on the sea as an example, the imaging effect of the imager during high-speed relative motion is simulated, and the degradation blurring is performed based on the clear actual images to verify the actual expansion effect.