In dim condition, by using a light sensor and infrared sensor can make up for human鈥檚 visual defects and make better observation of the object and background environment. If combining the image information from the two sensors at the same time and the same place, you can generate a new image with more information, precision and credibility; however traditional windows based weighted average fusion algorithm has some limitations, the paper put forward an fusion algorithm with an improved adaptive threshold based on windows in dim conditions. By contrast experiment, it proved that the value of entropy, average gradient and edge retention of the improved method have been all increased, and the effect of image fusion has also been changed better