A bracing device has a supporting module and a restraining element mounted around the supporting module to provide a restraining-supporting effect to the supporting module. The supporting module has at least one supporting element. The at least one supporting element is elongated and has an axial segment and two connecting heads. The axial segment has at least one concave edge and at least one loading section. The connecting heads are respectively formed on two connecting ends of the axial segment. The restraining element has at least one viewing hole aligning along the at least one loading section of the axial segment to enable a user to see the at least one supporting element of the bracing device via the at least one viewing hole without dismantling the restraining element.