The invention relates to a light spot image extraction method in a linear array APS (Auxiliary Propulsion System) sun sensor. Aiming at the characteristics of the linear array APS sun sensor and considering the interference of parasitic light, the influence of temperature to a device, secondary reflection of light in a cavity and other adverse effects, the method adopts dynamic gray thresholds, only grey scale values larger than the threshold part in a whole frame image are taken as extracted light spot data; meanwhile, effective images are provided to determine the thresholds which are taken as the criterion for distinguishing true and false light spots, and formed light spot images are screened; the light spot images are identified after being extracted, and unwanted images are picked by determining the positions of three straight slits and the sum of image grey scale values. According to the method, under the condition of high temperature, light spot images can be extracted quickly and effectively when the back noise of an image sensor is increased greatly; when secondary reflection exists on the surfaces of light slit glass and the sensor, even if false images appear in the images, true light spot data can still be identified, so that two-axis angle calculation in the later period can be accomplished conveniently.