In order to match the image motion produced by the movement of ground objects at a focal plane accurately,a calculation method for image motion speed is realized by taking the spacecraft location vector and velocity vector based on the WGS-84 coordinate system and the spacecraft attitude angle and attitude angular velocity based on spacecraft orbital coordinate system as input parameters.Firstly,a inertial coordinate system is selected flexibly based on the original calculation model of image motion speed to decrease the complexity of solving orbital inclined angels and descending node longitudes.Then,the cosine value of central angle between the spacecraft and the descending node is solved by using cosine theorem of spherical geometry to avoid the cumbersome solving process by judging the flight direction of a satellite and the latitude of sub-satellite point in an original calculation model of image motion speeds.Subsequently,the longitude and the latitude of sub-satellite point and the FLASH address are linked organically and skillfully to avoid the cumbersome lookup process when elevation data are visited.Then the feasibility of calculation model of image motion speed is evaluated by analyzing the impact of the residual error of image motion speed on the MTF of a camera.Finally,the whole process of calculating image motion speed is realized on DSP from TI.Analyzing processes and experimental results indicate that the residual error of calculation model is 0.15%,which results in a 1% decrease of the MTF.The calculation time of image motion speed is less than 2 ms.These data satisfy the system requirements of the camera on the calculation accuracy and computing time.