In a sensed image of long distance, the gray levels of target and background are hardly distinguishable, which results in a low-contrast image. Dim-target detection is always a difficult problem. The aim of this paper is to propose an anisotropic diffusion filtering algorithm based on partial differential equation to enhance the dim targets. The algorithm establishes a new filter model by improving the traditional P-M model based on the anisotropic diffusion theory. The proposed method adaptively performs the smoothing process in the faultless areas to make the background uniform, and performs the sharpening process in the variational areas to enhance dim targets. Simultaneously, we can select the smoothing and sharpening degree by adjusting the parameter K and w to satisfy different environments. Experimental results show the efficiency of the proposed diffusion scheme in dim-target enhancement with low-contrast.