The invention relates to an instantly opening and heating water-energy saving water heater system consisting of a cold water pipeline (1), a water heater (2), a heat-insulating water storage device (3), a hot water pipeline (4) and a hot water terminal (5). In the system, a new idea for solving residual cold water in a pipeline is utilized, thus a problem of waste caused by idle storage of the residual cold water is solved, and an ideal use effect of instantly heating while opening is achieved; in addition, the system has the characteristics of water, energy, time and worry saving, convenience and comfortableness. The instantly opening and heating water-energy saving water heater system is simpler, more reasonable, more convenient and more economical than other traditional systems for solving the problem of the residual cold water, such as a scheme of providing a high-power electric heating water faucet or adding an assistant pipeline for recycling the residual cold water. The instantly opening and heating water-energy saving water heater system disclosed by the invention improves the comfort level of using hot water actually, is suitable for an electric water heater, a gas water heater, a solar water heater or a combined system thereof and is also suitable for a newly assembled water heater system; or by means of the instantly opening and heating water-energy saving water heater system disclosed by the invention, the performances of old systems are improved and enhanced.