The utility model discloses a protector for a double-cavity proportioning valve when one cavity fails. When a double-pipeline braking system of an automobile normally works, a sealing ring at one end of a spacing sheath is sheathed on an O-shaped seal ring at one end of a seal assembly all along under the action of a spring in a valve body hole. The fluid pressure of an input hole and an outlet hole is spaced by the O-shaped seal ring of the spacing sheath. Each part of the A cavity and the B cavity in the valve body hole is in balance. When the A cavity system in the double-pipeline braking system of an automobile fails, the input fluid pressure and the output fluid pressure of the A cavity are equal to zero. A plunger piston and an O-shaped sealing ring which is sheathed on the plunger piston push the spacing sheath and the O-shaped sealing ring which is sheathed on the spacing sheath and compress the spring to move towards the direction of the A cavity to make the input hole and the output hole of the B cavity to form a liquid passage or a gas passage to protect the proportioning valve when one cavity fails. The principle is the same if the case is in inverse. The utility model is suitable for double-cavity liquid (gas) pressure loading sensing proportioning valves and non-loading sensing proportioning valves with various structures.