Based on statics of four-bar linkage roof-windows mechanism for multi-span plastic greenhouse, considering intensity requirement, installation error, boundary and other restrictions, optimizing model taking for minimal pneumatic cylinder output as object was developed and optimized results adopting standard pneumatic cylinder meet stability requirements. The optimum programme can be used to design pneumatic four-bar linkage roof-window mechanism of all kinds of multi-span plastic greenhouses by changing load and structure parameters. Running the new system indicates that the opening angle increases by 20° and its height increases by 28.2%, compared with traditional gear-rack operation system. It only takes 9.3 to 11.7 seconds to arrive at maximal angle.