The principle of the phenomenon that the infrared imaging system may defocus with the change of temperature is analyzed, and a linear infrared athermalization detection system is proposed. The characteristics of linear infrared imaging are analyzed. To solve the problem that the traditional methods of SMD, TenenGrad, Laplacian are prone to be disturbed by the stripe noise of the linear infrared system and considering the shape of the target plate, a method of definition evaluation based on the improved Sobel operator in seven directions is proposed. The experimental results show that this method can effectively overcome the interference of the stripe random noise of the linear infrared imaging system on the definition evaluation factor. Comparing with that of the traditional algorithms, the response curve of the proposed algorithm has obvious variation tendency and small fluctuation. It can better meet the monotonic and unbiased requirements of autofocusing.