The invention relates to a method of a vacuum pressure casting machine and a device thereof, comprising a movable shell and a sliding plate, a fixed shell and a bottom plate, a sealing part, a vacuum pump, a pressure gauge, a vacuum energy storage tank and the like. After the sliding plate rises, the movable shell and the fixed shell are sealed to form a sealing cavity (A) which is connected with the vacuum energy storage tank and the vacuum pump by a pipeline and is provided with a valve; the bottom plate is provided with a material inlet which is connected with a funnel-shaped charging barrel that is internally provided with a pressure injection piston; a mould is arranged inside the sealing cavity (A); and when in work, an opening of the charging barrel is plugged by the pressure injection piston, melt liquid is added into the funnel-shaped charging barrel, the melt liquid is quickly sucked into a die cavity by vacuum when the pressure injection piston is lifted, and the pressure injection piston goes downwards to pressurize the melt liquid so as to lead metal liquid to be solidified under the pressure. After solidification, the sliding plate goes downwards, an ejection oil cylinder is used for pushing an ejector rob to eject a workpiece. The workpiece is charged and molded under the normal state of vacuum (negative pressure) and is solidified under the pressure, thus eliminating or reducing air holes inside castings and ensuring the workpiece to be well charged and molded.