The invention relates to a bath water output control system and a water heater. The system comprises a controller, a water flow sensor connected with the controller, a control assembly and a remindingdevice. The water flow sensor is used for being connected with a water outlet of the bath water output control system. The water flow sensor senses water flow information of the bath water output control system and sends the water flow information to the controller. The control assembly sends parameter information to the controller, wherein the parameter information comprises a current bath modeand the preset water output total flow. When a current bath mode is a bathtub bath mode, the controller sends a reminding signal to the reminding device if the controller detects that the water flow information is larger than or equal to the preset flow. The preset flow is obtained according to the difference between the preset water output total flow and the preset water flow. The reminding device outputs the reminding information according to the received reminding signal. According to the system, the bathtub bath mode is put forward, the requirement of users for bathtub bath is met, when the current mode is the bathtub bath mode, when water is full, reminding is carried out, and convenience is improved effectively.