The invention relates to a preparation device of a composite separation membrane. The preparation device comprises two material winding and unwinding frames, a reaction frame, two solution tanks, a liquid level control roller group, two thin film drying boxes and a post-treatment water tank, wherein the two winding and unwinding frames are respectively arranged at the front end and the back end of the device, the solution tanks and the liquid level control roller group are mounted in the reaction frame, the two solution tanks are respectively arranged on the two sides of the liquid level control roller group, and the two thin film drying boxes are respectively arranged on the two sides of the post-treatment water tank; and the composite separation membrane sequentially connects the first material winding and unwinding frame, the reaction frame, the first solution tank, the liquid level control roller group, the second solution tank, the first thin film drying box, the post-treatment water tank, the second thin film drying box and the second material winding and unwinding frame through a transition guide roller. The device provided by the invention has the characteristics of compact structure, capability of precisely controlling the content of a solution adsorbed in a support body and relatively high drying efficiency, and the obtained composite separation membrane has good physical properties.