The invention discloses a chloramine preparation device capable of automatically adjusting the reagent dosage according to the variable temperature and a preparation method thereof. The device comprises a reagent storage system, a chloramine generation system, a chloramine storage system, a control system, and a support; the reagent storage system is composed of an ammonium chloride storage box and a sodium hypochlorite storage box, the control system comprises a PLC master control box, a temperature sensor, an electric valve, a meter pump, and a liquid level sensor, wherein the temperature sensor, electric valve, meter pump and liquid level sensor are all connected to the PLC master control box; the chloramine generation system is composed of a chloramine reaction box and an electric stirrer, the chloramine generation system and the chloramine storage system are connected by a stainless steel pipe, moreover, an electric valve is arranged to control the communication state between two systems; and the chloramine storage system adds the chloramine into a water supply network through the meter pump. The provided device can be used to continuously prepare chloramine disinfectant and quantitatively add chloramine disinfectant for a city/town water supply plant, the device is not affected by the temperature change, the concentration of the prepared chloramine is stable, the automation degree of preparation and addition is high, the operation is simple, and the maintenance is convenient.