The utility model relates to an automatic valve triggered to act by compression bar instability under the working condition of excessive pressure. The automatic valve mainly comprises a pneumatic actuator, a switching valve and a two-position multichannel reversing valve controlled by compression bar instability, wherein the reversing valve mainly comprises a bar cap, a bar cage, a slender bar, a valve body, a valve spool component, an end cover and the like and is designed based on the principle of compression bar instability, the valve spool component bears pressure load from a pipeline, the load is loaded on the slender bar, the valve spool component is placed at a first working position by the aid of axial thrust of the slender bar when action force generated by medium pressure in the pipeline is lower than the critical load of the slender bar, and the automatic valve can be placed at a normal valve position by the aid of a power air source flowing through an internal channel of the reversing valve. When the action force generated by the medium pressure exceeds the critical load of the slender bar, the slender bar is instable, the medium pressure can push the valve spool component to act to a second working position, the pneumatic actuator can drive the switching valve to automatically act, and the normal valve position of the automatic valve is changed into an abnormal valve position, so that the valve is automatically opened or closed under the working condition of the excessive pressure.