The invention relates to an efficient water-saving and energy-saving device of a water heater based on a PLC (Programmable Logic Controller). A constant-temperature control device is respectively connected with a cold water inlet pipe and a hot water outlet pipe through a water quantity adjusting valve, a water inlet is connected with a user water outlet pipe; a cold water outlet of a cold water quantity adjusting valve is communicated with a cold water inlet of a hot water quantity adjusting valve and then is connected with the cold water inlet pipe of the water heater; the cold water quantity adjusting valve and the hot water quantity adjusting valve are respectively connected with a valve core rotation transmission device; a water pump is connected between the hot water quantity adjusting valve and a hot water inlet pipe; the outlets of the hot water inlet pipe and the user water outlet pipe are respectively provided with temperature sensors; and the temperature sensors, the valve core rotation transmission device and the water pump are respectively connected with a PLC automatic control device. According to the device disclosed by the invention, by additionally arranging the water-saving and energy-saving device on the existing water heater, cold water and hot water are used so as to achieve the aim of saving energy and reducing emission before and after the water heater is used; by additionally arranging a thermostatic valve, the water temperature is stable when the water heater is used, so that the energy is saved and the comfort level of use is improved; and the device is simple and feasible.