For the problem of ship target fast detection and speed estimation of space-flight image, this paper firstly analyzes the feature of sea-land background image. Based on the pixel point gradient extraction and edge detection technique, a detecting algorithm based on gradient map three filtration is proposed. This algorithm can realize the segmentation and extraction of sea area and ship target. Then, by the Fourier fitting model, this paper puts forward a period estimation method of Kelvin wake to calculate the speed of ship. The error range and error source are analyzed in the end.