Aiming at the problem of low detection precision and poor effect of single frame infrared dim target detection algorithm, a novel algorithm of single frame infrared small target detection based on improved Sobel is proposed. According to the characteristic of Sobel operator edge detection, firstly, the image median filtering can improve the dim small target signal-noise ratio, and then use the Sobel operator and the proposed detection template to deal with the image convolution, finally detect the infrared dim small target after median filtering. The effectiveness of the improved algorithm is analyzed theoretically and compared with the centroid detection algorithm and the detection algorithm based on local mean and Top-Hat. Experimental results show that the proposed algorithm is more accurate than other detection algorithms in the experiment, and the detection effect is more obvious.