A simply-supported fixture for a circular plate, including: an opening end cover, circular sleeve, base, and semicircular-ring convex platform. The base has a positioning step for guidance, the circular sleeve is concentrically installed on the step of the base, and the opening end cover is concentrically installed on the circular sleeve and connects to the base through a reaming hole bolt. A circular plate disposed between the opening end cover and base, and opposite sides of the opening end cover and base are provided with concave platforms. Two semicircular-ring convex platforms of the same diameter are concentrically installed on the concave platforms, the reaming hole bolt is tightened with force, and the circular plate is supported and fixed on the fixture. The circular plate and the semicircular-ring convex platform on the opening end cover and base, are in line contact wherein lines are parallel and concentric having the same radius.