The invention discloses a supervising system and method for city free bicycles and belongs to the technical field of supervision of the city free bicycles. The supervising system comprises a control center and the multiple bicycles. One of tires of each bicycle is internally provided with a pressure sensor which is connected with a first wireless transmission module. A protection box is installed below a seat cushion of each bicycle and internally comprises a second power module, a single-chip microcomputer, a GPS module and a second wireless receiving and dispatching module. An automatic lock is arranged at a bicycle brake of a back wheel of each bicycle, and the automatic lock, the GPS module and the second wireless receiving and dispatching module are connected with the single-chip microcomputer. The first wireless transmission modules are connected with the second wireless receiving and dispatching modules, and the second wireless receiving and dispatching modules are connected with the control center. People can have access to the control center through mobile phones. By arranging the pressure sensors in the tires of the bicycles and arranging the automatic locks at the bicycle brakes, the suitable bicycles are provided for users according to the conditions of the bicycles, and the problem that the riders cannot get the conditions of the bicycles in time is solved.