Abstract: A water temperature detecting and controlling device may be controlled automatically or manually so as to regulate the ratio of the cold to hot water in a prior faucet, so that the temperature of the water flowing out of the faucet may be controlled. The structure of the device is comprised primarily of a motor with a shaft which is fixedly engaged to the flow by-pass rod of the water mixing structure, while said motor is received within a rotary head which is fixedly screwed to a flow by-pass rod; therefore, the rotary head may be rotated by hand to drive the motor and the flow by-pass rod or to provide a water flow to drive the motor to rotate and then further to drive the flow by-pass rod to rotate, this similarly achieves the effect of distributing the cold and hot water. Additonally, the rotation of the motor is controlled by a microprocessor.