A mechanical bicycle automatic speed changer comprises an input end outer shell, an output end outer shell, an inner layer shell, a transmission mechanism and a variable speed adjusting device. The input end outer shell is internally provided with an input shaft. One end of the output end outer shell is in threaded connection with the input end outer shell, and the other end of the output end outer shell is in spline connection with an output shaft. The inner layer shell is arranged inside a shaft outer shell formed by the input end outer shell and the output end outer shell and contains the transmission mechanism and the variable speed adjusting device arranged on the outer side of the transmission mechanism in a sleeved mode. The transmission mechanism further comprises a first conical body, a second conical body, a transmission steel belt and a stable small shaft, wherein the transmission steel belt is arranged on the outer edge of the first conical body and the outer edge of the second conical body, and the stable small shaft is arranged on one side of the first conical body and one side of the second conical body. The variable speed adjusting device further comprises a first adjusting sleeve, a second adjusting sleeve, a thrusting spring and an adjusting pushing ring. The mechanical bicycle automatic speed changer cannot be damaged easily, the reliability is enhanced, the transmission ratio can be automatically adjusted, the riding strength is reduced, and the user using experience is enhanced.