A calculating model based on difference method was proposed to calculate the image motion velocity vector to campensate precisely the image motion of a space camera. Firstly, the earth imaging model was established by utilizing coordinate transformation,and the relative motion between the image point and the image plane was analyzed in vertical, roll and pitch attitudes according to the states of the image motion. Then, a method was proposed to calculate the image motion velocity vector according to the law of the relative motion between the image point and the image plane. Finally, the proposed method was compared with the coordinate transformation method. The result shows that the error between the two methods is within 0.1%, which proves the efficiency of this method. The proposed method overcomes the complex compulation from traditional methods and can be used in calculating the image motion velocity vectors for other space cameras.