The invention relates to a flame detection method adaptive to large outdoor space. The method comprises the following steps: converting a video frame image to a grayscale image, and taking the mean image of previous N frames (N is greater than 50) as a temporary background frame; making difference between the current frame and the temporary background frame, and then carrying out mask treatment to obtain a mask binary image; carrying out binarization on the current frame to obtain a brightness target, making difference between the current frame and the background frame and carrying out binarization on an Otsu self-adaptive threshold value to obtain an object target, and getting intersection of the brightness target and the object target as a target binary image, wherein the communicated areas in the target binary image are the suspected flame target; marking the communicated areas and storing information of the communicated areas; establishing a target chain table for each communicated area, and chaining the target chain table; calculating information of a traced moving target and determining whether the target is flame or not according to flame alarm probability. The method provided by the invention is adaptive to changes of climate and illumination, can be adaptive to indoor and outdoor flame detection, and can immediately and accurately alarm an incipient fire.