It is found that the vacuum backpressure affects the flow of background gas into a Hall thruster channel. The flow of background gas can affect the ionization of propellants and the conductivity of electrons, and thereby affect the physical process and discharge characteristic of Hall thrusters. In order to investigate these effects, the atomic and the ionic emission spectra in the discharge channel and the ions energy distribution are measured under different vacuum backpressures. The experimental results indicate that the increase of vacuum backpressure can reduce the electron temperature and ionization efficiency of propellants and a new ionization region appears near exit plane in the channel; and moreover, it is found that the new ionization region penetrates into the channel more deeply under the higher vacuum backpressure case.