This invention discloses a valve including a body, an upper membrane, a lower membrane and a positioning block. The body has an upper channel, a lower channel, an accommodation space and a valve groove. The accommodation space defines an upper opening and a lower opening for respectively communicating with the upper channel and the lower channel. The valve groove is near the intersection of the lower channel and the accommodation space respectively communicating therewith. The upper and the lower membrane block the upper opening of the accommodation space and the lower opening. The positioning block is disposed between the upper membrane and the lower membrane and is able to push the lower membrane into the valve groove and abut against a sidewall of the valve groove. The valve is able to block the flow of water in the valve when the water flow is needed to be cut off.