The invention discloses an ultrahigh vacuum automatic pressure relief valve, which comprises a valve body, a valve plate, a valve stem, a valve cover, an adjusting screw rod and an adjusting spring, wherein the valve body is provided with a valve body tube and a valve seat; the lower end of the valve body tube is connected with the valve seat, and a part of the valve body tube close to the valve seat is provided with holes for pressure relief and evacuation; a sealing member is arranged on the valve seat; the valve plate can move in the valve body; when the bottom of the valve plate is pressed on the sealing member on the valve seat of the valve body, the valve plate and the sealing member are in vacuum seal; the lower end of the valve stem is connected with the upper part of the valve plate so as to drive the valve plate to move in the valve body tube; the valve cover seals the upper end of the valve body tube and has a central hole; the adjusting screw rod is in threaded connection with the central hole of the valve cover and has a central hole; the upper end of the valve stem runs through the central hole of the adjusting screw rod, and the valve stem can move in the central hole of the adjusting screw rod; and the adjusting spring is sleeved on the valve stem and is limited between the valve plate and the adjusting screw rod, so the adjusting screw rod can be rotated to adjust the elasticity of the adjusting spring.