In this essay, the gas flow state and emergence of excitated liquid inside the ejecting tube of the two stage water ring vacuum pump with air ejector are analysed and calculated by the theories and formulas of unary isentropic flow and the characteristical dimension design calculation formulas for the ejector pump are given in accordance with the test and design experiences. It’s well known that the air ejector has the features such as simple structure, reliable work, light weight, no mechanical moving parts, i.e., no friction, etc. therefore, it’s widely used together with the water ring vacuum pump to form a vacuum air extraction pack to increase the air pumping capacity. For example, when only use the two stage water ring vacuum pump, the work situation point often used is 100-60mbar of suction pressure ,whose lowest pressure is 30~25 mbar (compensation water temperature is 15~20). when used together with the air ejector, the lowest suction pressure is 4~5 mbar. When the suction pressure is 13~15mbar, the air pumping speed is still able to reach 55~60% of the maximum pumping speed of the two stage water ring vacuum pump.this is quite favourable to the work situation which pumps corrosive gas and gas with trace impurity or much steam and requires a larger pumping speed when the suction pressure is 13~15 mbar. Hence, this kind of machine pack is widely used in chemical industry, pharmaceuticals, petrochemical, metallurgy, light industry and so on. Many factories and companies in the world all produce the machine pack formed by two stage water ring vacuum pump with air ejector. The maximum specification has already reached 60~70m@super 3@/min. for example, the pumping speed of glrd 3200 type two stage water ring vacuum pump with air ejector produced by general company in U.K. is 60 m@super 3@/min (suction pressure is 15 mbar). This essay will theoretically analyse and calcucate the gas flow in the ejector pump of such kind of vacuum air pumping machine pack and introduce the calculation ascertainment of its characteristical dimension.