The invention discloses a supporting device for repairing an ancient building. The device comprises a supporting upright post, a supporting cross rod and a supporting cross beam, the supporting crossbeam is arranged on the supporting upright post, the supporting cross rod is arranged on the supporting cross beam, the supporting upright post is provided with a decorative upright post, the supporting upright post is provided with an annular convex part, the convex part is provided with an annular limiting block, the decorative upright post is provided with an annular groove, the annular grooveis matched with the annular convex part, and the annular groove is provided with an annular clamping groove. A construction method of the supporting device comprises the following steps of a, preparing ancient building measurement and construction; b, mounting a supporting structure; c, mounting a fixing mechanism, the decorative upright post and a decorative rod; d, mounting a clamping rod and alimiting rod; e, paving a roof supporting frame; and f, installing a wood board. The ancient building can be reinforced, and the construction method of the supporting device can reduce damage to the ancient building in the construction process and keep the original appearance of the ancient building.