According to the characteristics of sky and earth background contour line such as fluctuation,blurring edge and slow change grey value of contour edge,grey threshold method is adopted to extract sky and ground background edges. It can provide technology support for subsequent sky background image segmentation and air target detection. The algorithm has the characteristics of easy to realize,few calculation,fast operation speed,precision location and low noise,which compensates the shortcomings of traditional image edge extraction algorithm. With background video images recorded from long range such as sky and surface,sky,surface and island,sky and mountain,the image processing result contrast analysis of the detection algorithm and other algorithm is proposed.