1. A method for fast multiple reference frame motion estimation, which is used to code an image frame sequence including a current frame Fn and multiple previous reference frames Fn−1, Fn−2,. .. , Fn−(k−1) and Fn−k positioned rearward in time with respective to the current frame Fn, where n and k are an integer, the frames being divided into a plurality of blocks, the method finding motion vectors for a block in the current frame referring to the multiple reference frames, the method comprising: a block-matching step, which performs a special block matching to find a plurality of motion vectors for blocks of each frame referring to a previous one frame; a motion vector composing step, which obtains composed motion vectors of the current frame referring to the reference frame Fn−k for the block, wherein 
 MVn ( i )= MVn ( i− 1)+ Mvn −( i− 1)(1) where Mvn(i) is the composed motion vectors of the current frame referring to the reference frame Fn−i, MVn(i−1) is the motion vector of the current frame referring to the reference frame Fn−(i−1) and MVn−(i−1)(1) is the motion vectors of the reference frame Fn−(i−1) referring to the reference frame Fn−i, i is an integer and 2≦i≦k; a selecting step, which selects one with a minimum of cost function from the composed motion vectors Mvn(i) produced in the motion vector composing step; and a fine adjusting step, which finely adjusts the composed motion vector selected, thereby obtaining a final motion vector of the current frame referring to the reference frame Fn−i.