A vacuum condition controlling apparatus, the top of which is connected with an electron beam generating instrument. The apparatus is rotationally symmetric, comprises the following parts deployed outward from the central axis: the central channel, the first pumping channel, the gas supplying chamber and the at least one pumping chamber. A pressure limiting aperture is deployed near the outlet of the central channel, for keeping the pressure difference between the central channel and the outside environment, and allow the electron beam to go through the central channel; the first pumping channel is connected to the central channel to pump the central channel; the top of the gas supplying chamber is connected to the gas supplying channel to supply gas to the area between the specimen and the apparatus; the top of the second pumping channel is connected to the second pumping channel, to pump the area.