Vacuum die casting is the optimal method to produce high quality aluminum alloy components. At present, there are still very few systematic studies on vacuum die casting theory and equipment design. On the basis of the existing theories of the vacuum die casting pumping and venting systems, a simplified model is established in this research. The model has an aggregate unit consisted of vacuum pump + buffer tank and a cylindrical container (including the shot sleeve, cavity and exhaust channel). The theoretical analysis is carried out between the cavity pressure and the pumping time under different volume models. An auxiliary system for high vacuum die casting is designed based on the above analysis. This system is composed of a vacuum control machine and a new vacuum stop valve. The machine has a human-computer control mode with programmable logic controller(PLC) + touch screen and a real-time monitoring function of vacuum degree for buffer tank and die cavity. The vacuum stop valve with the compressed gas + piston rod + labyrinth groove structure can realize the function of whole-process vacuum venting. The new system shows great advantages on vacuuming the cavity with a much faster speed by making tests on an existing die casting mold and a 250 t die casting machine. A die cavity pressure less than 10 kPa can be reached within 0.8 s in the experiment and the porosity of castings can be greatly decreased. The systematic studies on vacuum die casting theory and equipment have a great guiding significance for high vacuum die casting, and can also be applied to other high vacuum forming in related theoretical and practical research.