A webbing height adjustment device is applied to connect to a webbing of a seat belt system. The webbing height adjustment device has a top ring member, a locking assembly, and a positioning member. The locking assembly is moveably disposed outside the top ring member. The positioning member is disposed on the side surface of the top ring member, is clutched with the locking assembly, and has a positioning element and at least one combining portion. The positioning element is disposed on the top ring member. The at least one combining portion is disposed on the positioning element and selectively and magnetically attracts the locking assembly for positioning. The webbing height adjustment device is easy to operate and automatically locks the webbing. The webbing height adjustment device can be adjusted continuously and occupies little space when not in use.