A method of making a strain wave includes the steps of a) providing a circular spline, a flexspline meshed with the circular spline, and an initial wave generator having an initial outer profile of a standard ellipse with a perimeter S0, and b) producing a modified wave generator rotatably fitted within the flexspline and having a modified outer profile with a perimeter S. A difference ES between the perimeter S of the modified outer profile and the perimeter S0 of the initial outer profile satisfies the equation ES=S−S0=0.1 m to 0.8 m, wherein m is the modulus of the flexspline. Through a special parameter design of the modified wave generator, the meshing ratio between the circular spline and the flexspline is increased, thereby improving the transmission accuracy and reducing the average load.