A warning device for bicycle mainly includes a magnetic and a coil mechanism. The magnetic mechanism includes a magnetic element assembly, a mounting module, and a holding module movably connected to the mounting module for holding the magnetic element assembly therein. The coil mechanism includes a housing, a coil assembly, a rectifier unit, a plurality of light-emitting elements, and a connecting module. The coil assembly, the rectifier unit, and the light-emitting elements are electrically connected to one another and then received in the housing. The mounting module of the magnetic mechanism is fixedly mounted on a fixed part of a bicycle close to a wheel thereof, and the connecting module of the coil module is fixedly connected to two spokes of the wheel, such that the magnetic element assembly and the coil assembly are located at corresponding radial positions relative to the wheel.