A pressure-activated parachute opening device for a water rocket, comprising a connection base, an elastic fixed rope, and a piston-spring mechanism; the side of the connection base is longitudinally equipped with a piston-spring mechanism and a convex There is a carbon fiber anchor bolt on the other side; one end of the elastic fixed rope is fixed to the anchor bolt, and the other end is connected to a collar; the piston-spring mechanism includes a soft air joint, a cylinder-piston assembly composed of a cylinder and a piston , extension spring, extension spring fixed hanging rod, extension spring movable hanging rod, extension spring movable hanging rod limiter, carbon fiber latch, the fixed hanging rod is fixed on the upper part of the outer wall of the cylinder, and the movable hanging rod is connected to the working end of the piston; The spring is hooked between the fixed hanging rod and the movable hanging rod; the limiter is arranged at the lower part of the cylinder; the upper end of the bolt is fixed on the movable hanging rod, and is compatible with the collar. The device of the invention satisfies the design requirements of independent work, strong reliability, high action triggering precision, moisture resistance and water pressure impact resistance.