Small reactors have received more and more attention for their high safety, reliability, low power density, and short construction period. And the gas-steam pressurizer is widely used in small reactors due to its characteristics of simple structure, saves the heating and spray equipment, and prevents the coolant from boiling. The gas-steam pressurizer is a pressure control equipment for the reactor coolant system, and its characteristic of transient response is an important factor that affect operation stability of nuclear reactor systems. An experimental system was established to study the effect of pressure response for an insurge transient and influence factors were analyzed quantitatively. Experimental investigation shows that for the gas-steam pressurizer, the increase of coolant loading capacity (insurge) can cause system pressure rising. And the change of system pressure has much consistency with the change of liquid level and gas space temperature. The liquid phase exists temperature fluctuations and overall shows a downward trend during the insurge transient. And there exists a temperature gradient from bottom to top in the pressurizer liquid phase region during the insurge transient. The change of water vapor quantity curve is the oscillating curve during the transient and water vapor quantity is in a decreasing trend overall during the insurge transient. What’s more, the experiments also analyzed the pressure response and temperature response during the insurge transient.