A heater for shrink fit chucks has a heating cover, a blocking piece, and an adjustable blocking set. The heating cover has a first through hole, a second through hole, and a heating space disposed within the heating cover, extending along an extending direction to form the first through hole and the second through hole, and communicating with a heat gun. The blocking piece is detachably connected to the heating cover to block the first through hole and has a communicating hole capable of communicating with the first through hole. The adjustable blocking set is connected to the heating cover and has two blade assemblies and a switching component. Each blade assembly has blades disposed annular along the extending direction. The switching component is rotatable and capable of pushing the blades to move toward or depart from each other for blocking the second through hole.