Multi stage safety injection device and passive safety injection system having the same

Disclosed are a multi-stage safe injection device and a passive safe injection system having the same. The multi-stage safe injection device comprises a safe injection tank which is formed to receive cooling water to be injected into a reactor vessel by a gravity head so that the cooling water can be injected into the reactor vessel in a multi-stage, when there is an accident causing a pressure or a water level of the reactor vessel to be lowered; a pressure balancing pipe which is connected to the reactor vessel and the safe injection tank to form pressure balance between the reactor vessel and the safe injection tank; a safe injection pipe which is connected to a lower end of the safe injection tank and the reactor vessel to inject the cooling water into the reactor vessel when the pressure between the reactor vessel and the safe injection tank is balanced; and a flow adjusting pipe which is connected to the safe injection pipe, is extended to the inside of the safe injection tank from the safe injection pipe to be connected to the safe injection pipe, and has a safe injection opening through which the cooling water is injected in the predetermined heights so that the flow of the cooling water injected into the reactor vessel can be gradually reduced as the water level of the safe injection tank is lowered.