The invention discloses a water leakage detector, mainly comprising a shell, a signal collecting device and a switch valve. In the invention, a piston of the water leakage detector is provided with a magnet, and a solenoid valve is arranged outside a water inlet horizontal pipe cavity of the shell and is opposite to the magnet sheathed in the piston, so the solenoid valve can sensitively detect the position change of the magnet in the piston and make a sensitive response to small change of water flow; and once the water pressure in a water pipe changes, the piston can move so as to drive the magnet to move, so the solenoid valve can transmit the change of an electrical signal caused by movement of the magnet to a warning device to give out the alarm. In addition, the water leakage detector is provided with the switch valve composed of a piston post, a waterway nut, a reset handle and the like; and a pathway of the water leakage pipe can be turned on or cut off so as to turn on or block up water flow by means of combination or separation of the solenoid valve and the reset handle on the switch valve,, thereby effectively avoiding further water leakage phenomenon.