Vertical two boards extrusion molding machine as compared with similar products, has some advantages as low consumption, small footprint, high product performance, low material consumption, and can produce large parts, and etc. Yet its main oil cylinder was often partially loaded because of its working principle. Aiming at this problem, the improved design of the structure, size of hydraulic cylinder seal was carried out, and the problem of the partial load was solved. The durability of oil cylinder is greatly improved, which also provide a reference for design of the main oil cylinder of the machine.