The invention discloses a water tap lock and a key for opening the water tap lock. The water tap lock comprises a locking cam and a lock cylinder; the side of the locking cam is provided with a locking hole for inserting the key; the lock cylinder is positioned in a cavity inside the locking cam; the inner wall on one end part of the lock cylinder is provided with internal threads matched with external threads on the outlet end of the water tap; a cushion block is arranged between the top of the lock cylinder and the locking cam; the center of the cushion block is provided with a central through hole for receiving a first magnetic block; an opening groove for placing the first magnetic block is arranged on the joint of the end part of the lock cylinder and the cushion block; the height ofthe opening groove is smaller than that of the magnetic block; and the position corresponding to the first magnetic block of the key is also provided with a second magnetic block which is repelled bythe polarity of the first magnetic block. The key comprises an upper key piece and a lower key piece, wherein the upper key piece and the lower key piece are correspondingly provided with section slots; and two section slots are matched to form a section cavity for receiving the second magnetic block. The water tap lock can better control the water consumption of water taps in public places, and can effectively save water consumption; in addition, the key matched with the water tap lock can easily and conveniently open or lock the water tap lock, is convenient to use and has a simple structure.