When the heating is turned on, the water temperature keeps dropping. Does anyone know what the problem is?

Personal judgment should be that there is something wrong with the thermostat. The function of the thermostat is to automatically adjust the amount of water entering the radiator according to the temperature of cooling water, and change the circulating range of water, so as to adjust the cooling capacity of the cooling system and ensure the engine to work in a suitable temperature range. The thermostat must be kept in good technical condition, otherwise it will seriously affect the normal work of the engine. Such as a thermostat main valve.