The external shading system is built at the place where up the roof of the greenhouse. This system has a set of independent rectangular frameworks on the upper side of the greenhouse, and the operation principle is almost the same of the internal shading system. The system motor drives the transmission rod to operate, and the gear/rack on the transmission rod changes the circular motion into a linear motion. The push-pull rod connected to the rack drives the aluminum alloy movable push rod to move parallel on the curtain line through the cross connection, and the aluminum alloy movable push rod pulls one end of the curtain to slowly unfold and gather, and all the unfolding and closing positions respectively touch the opening and closing limiter switch, and then the motor stops and the operation ends.