An automatic speed change structure of a wind-powered water pumping device is disclosed. The automatic speed change structure is disposed at the top of a tower on which the wind-powered water pumping device is mounted so as to make use of wind power to drive a power transmission shaft arranged in in the tower to produce kinetic energy for a subsequent water pumping operation. The automatic speed change structure comprises a hollow body, a rotary blade set, a speed change gear set, an engagement parts, and an elastic support element. This invention uses the speed change gear set equipped with a first and a second driving gears and a first and a second driven roller set with different speed ratio relationship, in conjunction with the elastic support element supporting a shaft of the rotary blade set, to drive the first driving gear to set the power transmission shaft in an accelerated operation condition when the wind is weak; and when the wind speed increases, the shaft is inwardly retracted to allow the second driving gear to be driven so as to set the power transmission shaft in a speed-reduced operation condition, thereby maintaining the operation of the power transmission shaft in a stable and uniform speed condition to provide the advantages of increased structure service life, automatic change of rotation speed of the power transmission shaft according to the wind speed, and enhanced water pumping efficiency.