The invention discloses a self-locking type mechanical ball float valve device and a self-locking method of the self-locking type mechanical ball float valve device. The self-locking type mechanical ball float valve device comprises a valve body. A sealing sheet is arranged on the inner side of the valve body so that an inner cavity of the valve body can be divided into a liquid inlet cavity, a control cavity and a liquid outlet cavity. The liquid inlet cavity and the liquid outlet cavity are located on the same side of the sealing sheet. The control cavity is provided with a through hole communicating with the liquid inlet cavity. The control cavity is provided with a flow discharging opening. The flow discharging opening is opened or closed, so that the sealing sheet moves towards the control cavity or towards the liquid outlet cavity, and therefore the liquid inlet cavity and the liquid outlet cavity are isolated or communicated with each other. According to the self-locking type mechanical ball float valve device and the self-locking method of the self-locking type mechanical ball float valve device, the liquid inlet cavity is located on the eccentric side of the sealing sheet, the area of the liquid outlet cavity is larger than that of the liquid inlet cavity, and therefore the valve body can be suitable for larger flow; besides, the pressure difference between the two sides of the sealing sheet to be larger, and sealing is better facilitated; in addition, the sealing sheet is more sensitive and capable of rapidly cutting off liquid feeding; and after the water level reaches the highest water line, water feeding is stopped more safely. The self-locking type mechanical ball float valve further has the advantages of being large in flow, simple in structure, reliable in self-locking performance and convenient to install.