In order to diagnose precisely the electron number density of microwave plasma jet in vacuum environment, the integrative probe whose function is the same as that of emissive and Langmiur ones was applied to measure the plasma space potential and its current and voltage property when the probe is charged by the electric power of emissive probe and scanning electric power separately. According to the plasma space potential, saturated current on the current-voltage curve of plasma can be determined, which can be used to estimate the number density of electrons. The experimental results show that the number density of electrons in microwave argon plasma jet in vacuum environment is 1×1016—7.2×1016/m3, when the vacuum pressure is 3.2 and 5.4 Pa, the microwave output power is under 60 W and gas mass flow rate is in the range of 42—106 mg/s.