A large L-shaped plate automatic packaging machine based on PLC was designed according to the special needs of large plates envelope. The functions and requirements of movements for back closure, angle fold and dispensing side seal were analyzed, three-dimensional models of taking the film, back closure, square plug sealing and dispensing were set up by Pro/E software, and then the key back closure and corners seal plug seal mechanisms were optimized to complete the design of large plates automated packaging machine. The packaging machine could realize complete envelope for large sections of two different sizes. The machine solved the corners neatly packed puzzle of big plate automation envelope with corners plug seal manner, and in addition improved the efficiency and envelope effect of big plate packaging machine. The plates of different size could be coated by the machine and it was easier for replacing components, it showed that the machine had a good application prospect and high practicability.