The utility model relates to a three-stage synchronous telescoping mechanism of a crane boom, belongs to the technical field of a crane, and aims at providing the three-stage synchronous telescoping mechanism of the crane boom which is simple and reasonable in structure, simple to operate, and safe and reliable to use, and achieves synchronous telescoping of three-stage telescopic booms by a single cylinder. The utility model adopts the technical scheme that the three-stage synchronous telescoping mechanism comprises a basic boom, a first-stage telescopic boom, a second-stage telescopic boom and a third-stage telescopic boom, wherein the first-stage telescopic boom, the second-stage telescopic boom and the third-stage telescopic boom are orderly sleeved inside the basic boom; a telescopic cylinder is arranged on the basic boom; a cylinder rod of the telescopic cylinder is hinged with the tail end of the basic boom; a cylinder barrel is hinged with the tail end of the first-stage telescopic boom; a first semi-girder wheel is arranged at the tail end of the second-stage telescopic boom; a second semi-girder wheel and a third semi-girder wheel are arranged on a roller carrier at the front end of the cylinder barrel of the telescopic cylinder; and a second-stage semi-girder inhaul cable is wound orderly through the second semi-girder wheel, the first semi-girder wheel and the third semi-girder wheel; and two ends are fixed at the tail ends of the basic boom. The three-stage synchronous telescoping mechanism is mainly applied to the boom of a boom crane.