The invention relates to a die cutting device, in particular to a flat-press type pressure adjusting device of a die cutting machine and a method thereof. The pressure adjusting device of a die cutting machine comprises a connecting bar connected between a crank and a slide block; the connecting bar comprises two sections; the two sections of connecting bars are integrally docked with each other through a nut/screw rod structure; the device can adjust the pressure of the die cutting machine by adjusting the fastening length of the screw rod; the pressure adjusting method of a die cutting machine comprises the following steps: adjusting the height of a step by grinding a sub nut to eliminate an axial clearance between a main nut and the sub nut as a whole body and the screw rod. The invention provides a pressure adjusting device of a die cutting machine and a method thereof with accurate pressure adjustment.