The utility model discloses an automatic withdrawing/expanding mechanism for a folding boom of a crane. The automatic withdrawing/expanding mechanism comprises a connecting assembly, a hosting boom assembly, a bracket assembly and a withdrawing/expanding assembly, wherein the bracket assembly is connected with the hosting boom assembly through the connecting arm, and the withdrawing/expanding assembly is connected with the hoisting boom assembly and the bracket assembly through the connecting assembly, respectively; the connecting assembly comprises a pin shaft member and an ear plate member, the pin shaft member comprises a first pin shaft, a second pin shaft and a third pin shaft, the hosting boom assembly comprises a first-section boom, a second-section boom and a third-section boom, the first-section boom and the second-section boom are hinged through the first pin shaft, the second-section boom and the third-section boom are hinged through the second pin shaft, and a lower chord of the first-section boom is hinged with a tower crane through the third pin shaft. According to the automatic withdrawing-expanding mechanism, the withdrawing and expanding actions of the folding boom are flexible and smooth, the transportation size is small, and the crane can be conveniently and rapidly transported, mounted and dismounted in a narrow and small space.