Provided is a gap-type heating device for an upper guide bearing collar of a large hydroelectric generating set. The upper guide bearing collar is of a detachable structure. A heating assembly is composed of three segments of heating units which are separated by detachable gap adjustment sheets. The gap between the heating device and the upper guide bearing collar is designed to be 9-12mm, heat is transferred through heat radiation, and the surface of the guide bearing collar is effectively prevented from being burnt in the heating process. The whole heating device is closed, and the heating sheets are isolated from the outside, so that the heating sheets are prevented from scalding people. The insulation and grounding of the heating device are in line with the requirements. Thus, on-site unsafe factors are effectively reduced.