In order to locate the weak and small target accurately in the complex environment, according to the Gaussian shape characteristics of the target, the infrared dim small target detection algorithm based on intensity gradient mapping coupled with multi direction Median filter is designed in this paper. Firstly, according to the average intensity of infrared target in four different directions, the classical median filter was improved to effectively suppress the noise in complex background. Then, based on the central pixel of the small and weak target, the intensity information of the whole infrared image is obtained. The infrared image was divided into four sub blocks along the radius direction, and the polar coordinate system of each sub block was established to calculate its corresponding gradient value. According to the ratio of the maximum to the minimum gradient, the gradient information of the whole infrared image was obtained. Then, the intensity and gradient information were fused to get the background suppression image for enhancing the infrared dim target. Finally, the non-zero pixel mean value in intensity gradient mapping was used to calculate the threshold value for segmenting the background suppression image and locating the small and weak target accurately. The test data show that compared with the existing infrared dim small target detection technology, under the complex background interference, this algorithm has higher detection accuracy which can identify the target completely, and it presents a more ideal ROC curve