The invention provides a constant temperature water tank, a water heater and a water temperature control method of the constant temperature water tank. A mixing portion and a stirring portion are included; the mixing portion comprises a shell, a first liquid passing plate is arranged in the shell, the first liquid passing plate at least divides a cavity in the shell into a primary mixing chamber and a secondary mixing chamber, the first liquid passing plate is provided with a liquid passing channel, and the primary mixing chamber and the secondary mixing chamber are in communication through the liquid passing channel; the shell includes a first side and a second side which are opposite, one end of the first liquid passing plate is arranged on the first side, and the other end of the first liquid passing plate extends toward the second side; and the stirring portion includes a first mixing member arranged in the primary mixing chamber. According to the constant temperature water tank in the embodiment, water in the constant temperature water tank can be sufficiently mixed, and the water temperature of the discharged water of the constant temperature water tank is stable.