Abstract: For unclear underground monitoring video image and error target detection caused by low intensity of illumination, a lot of dust and strong light interference, an image preprocessing method for underground monitoring video was proposed. Histogram equalization algorithm is introduced to transform gray value from non-uniform distribution state to uniform distribution state and extend dynamic range of pixel gray value. Multi-features judgment method based on gray threshold, difference threshold and size threshold is used to identify spot. The experimental results show that the method can effectively enhance image contrast, improve image quality, and identify and filter spot, so as to improve effectiveness of target detection.