The invention relates to a reducing piled force-expanding type oil cylinder opening-closing device. In the invention, a short-
stroke large-cylinder-
diameter flat oil cylinder is connected between a
piston rod and a cross arm of a power oil cylinder, wherein the
piston rod of the flat oil cylinder is connected with the
piston rod of the power oil cylinder, and a close hole end cover of the flat oil cylinder is connected with the cross arm. The using method comprises the following steps of: filling hydraulic oil into an inlet-outlet oil hole of a piston cavity of the flat oil cylinder to ensure that oil enters the piston cavity and the
piston rod is in a fixed state; acting on the close hole end cover of the flat oil cylinder by the piston cavity after the oil enters the piston cavity to ensure that the close hole end cover pushes the cross arm and a pull rod pulls a
metal die core, wherein because the sectional area of the piston cavity of the flat oil cylinder is an odd
number times that of the piston cavity of the power oil cylinder, under the action of several times of pulling force, the
metal die core and an outer die of a
metal die form a die opening and core pulling
distance space; and then filling oil into the piston cavity of the power oil cylinder to finish the overall die opening and core
pulling action of the
metal mold. The invention is applied to a metal die core pulling and demoulding
casting process, reduces the shut-down waste caused by the braking of the metal die core, saves the resources and improves the working efficiency.