We have studied the motion law of spiral waves in two-dimensional excitable media. The motion of the wave front of spiral waves is divided into two parts, i.e., those along the tangent and the normal directions, and the motion equations in the two directions are obtained with the help of the perburbation method. When these equations are used to analyse the spiral tip, the basic motion law of the spiral tip and the structures of twisted rings and long twisted chains are obtained. When these equations are used to analyses the wave front and its disturbance, the result not only shows ordinary spiral shapes but also explains the super-spiral and the double-spiral structures.