A power generating device capable of outputting at a constant rotation speed is configured with a constant speed unit for transforming a varying input rotation speed into a constant output rotation speed, which comprises: a power source, an electric generator and a constant speed unit; wherein the constant speed unit further comprises: a first differential gear set, having two input ends and an output end being configured in a manner that one of the two input ends is connected with the power source and the output end is connected to a load; a continuously variable transmission (CVT) mechanism, configured with an input end and an output end in a manner that the input end is connected to the power source; a hydraulic torque converter, configured with an input end and an output end in a manner that the input end is connected to the load; a second differential gear set, configured with two input ends and an output end in a manner that the two input ends are connected respectively to the output end of the CVT mechanism and the hydraulic torque converter so as to perform a power composition process in the second differential gear set and then transmit the composite power from its output end to the other input end of the first differential gear set that is not connected to the power source.