The utility model relates to an additional device of the solar energy water heater, in particular to an automatic water adding device of the solar energy water heater. The utility model is characterized in that the utility model consists of a float mechanism, a conduct pipe and a water valve, wherein both ends of the conduct pipe are respectively connected with the float mechanism and the water valve, a sliding connecting rod is arranged in the conduct pipe, and the both ends of the connecting rod are respectively connected with the float in the float mechanism and the valve in the water valve; the float mechanism consists of a float chamber and a float, the top end of the conduct pipe is connected with the bottom part of the float chamber, and the top end of the connecting rod is connected with the float; the water valve is composed of a valve body, an inner valve chamber and a valve arranged in the water valve, the inner valve chamber is arranged in the valve body, the tail end of the conduct pipe is connected with the top part of the inner valve chamber, and the ball shapes valve in the inner valve chamber is connected with the tail end of the connecting rod. According to the water adding device implemented by the present proposal, the utility model has the advantages that the structure is simple, the installment, the operation and the maintenance are convenient and safe, the ware level can be adjusted arbitrarily and the price is inexpensive, therefore the utility model is suitable for the expansion use in the rural areas in particular as an effective and beneficial design.