The invention provides an overflow blocking safety valve, wherein when the overflow is generated due to the broken of a discharge pipe and the disconnection of a connection part, a ball attached on a magnet immediately blocks the outlet. The overflow blocking safety valve comprises a cylindrical barrel, an influx plug and a ball. One side of the cylindrical barrel is equipped with an outlet and an installation part. A plurality of guide members are formed on the installation part from the outlet to the inner side thereof. The flange part and the central protruding part of the influx plug are connected into one piece by a connection piece, so that an inflow path is formed between the flange part and the protruding part. A magnet is embedded in the protruding part so as to be connected with the cylindrical barrel in an opposite direction of the outlet according to a predetermined configuration. The ball installed inside the installation part is supported by the guide members and is attached to the protruding part by means of magnetic force. When the overflow occurs, the ball blocks the outlet.