The invention discloses a water temperature control system of an internal combustion locomotive, and belongs to the technical field of automatic control of internal combustion locomotives. The water temperature control system aims to overcome the problem that a current temperature relay is liable to be out of order, a temperature measuring circuit is used for optimizing a current water temperature relay of the locomotive, a U/F converter and relevant circuits are additionally arranged, a temperature signal of the temperature measuring circuit is transmitted to a current PLC of the locomotive, and no-contact control of the temperature control system is realized by the PLC. The control system comprises a PLC controller, and further comprises a voltage transformation circuit, a temperature sensor, a temperature measuring amplifying circuit and a U/F conversion circuit, wherein the voltage transformation circuit is separately connected with the temperature sensor, the temperature measuring amplifying circuit and the U/F conversion circuit; the temperature sensor is connected with the temperature measuring amplifying circuit; the temperature measuring amplifying circuit is connected with the U/F conversion circuit; and the U/F conversion circuit is connected with the PLC controller. The water temperature control system disclosed by the invention is suitable for being used by automatic control systems of various internal combustion locomotives.