The invention discloses an obstacle prewarning system for a bicycle, belongs to the field of prewarning systems and aims to solve the problem of how to assist a rider in knowing attributes of an obstacle in advance, distance between the obstacle and a bicycle body and speed of the bicycle body for the convenience of the rider to well make preparation for deceleration. The obstacle prewarning system structurally comprises an obstacle detection unit, a bicycle speed detection unit, a control unit, a wireless communication unit, an alarming unit, a server and a power source unit, the obstacle detection unit comprises an obstacle distance detection module and an obstacle recognition module, and the control module comprises a controller, a storage module and a data processing module which is connected with the obstacle detection module, the obstacle recognition module and the bicycle speed detection unit; the data processing module is connected with the controller which is connected with the server through the wireless communication module, and the alarming unit comprises a voice generation module and a voice playing module. Through the system, obstacle prewarning of a driver in advancecan be realized, so that convenience is brought to the driver for deceleration in advance.