The invention discloses an MOCVD (metal organic chemical vapor deposition) reaction device for preparing semiconductor epitaxial wafers. The reaction device comprises a gas inlet and a reaction chamber, wherein a top cover is arranged above the reaction chamber, the gas inlet vertically runs through the top cover, and the side wall of the reaction chamber is recessed inwards to form an inwards recessed annular structure; a substrate bearing seat capable of rotating axially is arranged below the reaction chamber, the outer diameter of the substrate bearing seat is smaller than the inner diameter of a cavity body, and a gap between the substrate bearing seat and the inner wall of the cavity body structure forms an exhaust channel; a fillet structure is arranged at the joint of the side wall and the upper edge of the substrate bearing seat, and the fillet structure and the inner wall of the cavity body structure form a Y-shaped funnel structure. According to the reaction device, the eddy current formed by upward rolling of gas in the reaction chamber is greatly inhibited and the stability of a gas field near the substrate bearing seat which rotates at a high speed is improved; due to the fillet streamline design, the distance between the bearing seat and the inner wall of the cavity body is reduced and optimized, so that the gas entering the exhaust channel from the reaction chamber hardly rolls upwards and the stability of gas flow in the reaction chamber and the uniformity of film growth are further improved.