The invention discloses a valve assembly. The valve assembly mainly comprises a spring actuator, hydraulic oil, a switching valve, a reversing valve, a hydraulic oil communicating pipe and a hydraulic oil storage tank. Medium pressure acts on a membrane of the reversing valve and is transferred to a slender rod for triggering the reversing valve through a valve element assembly, fixedly connected with the membrane, of the reversing valve. The reversing valve controls a drainage channel of the hydraulic oil. The Eula law of the slender rod instability is applied to triggering of the reversing valve. When the medium pressure reaches the triggering pressure of the reversing valve, the drainage channel of the hydraulic oil is opened through the reversing valve, so that the spring actuator loses the supporting function of the hydraulic oil and acts, and the switching valve in the closed state is opened, or a main valve in the open state is driven to be closed. The action of the valve assembly is free of driving of any power, sensors or instrument air, and the valve assembly can be used as an emergency drainage device or an emergency closing device.