In this paper, a generalized motion blurring model is constructed from the viewpoint of optical flow. Then based on the model, forward motion blurring kernel is deduced. The kernel provides a theoretical foundation for forward motion deblurring of high speed railway from image sequences. A fast method is also designed to estimate forward motion blurring kernel on this theory. Three specific problems are solved in this process. First, the analytical solution under quick motion estimation method is obtained. Next, the analytical solution under quick motion estimation method of planar scene direction is achieved. Lastly, the numerical calculation algorithm of forward motion blurring kernel is developed. Experimental results validate the proposed method.