A plasma column with length of several decimeters is generated in the upstream region of flowing gas (argon and trace air）at atmospheric pressure by using dielectric barrier discharge device in coaxial configurations. The discharge mechanism of the plasma column is investigated by optical method. The result indicates that a moving layer of light emission propagates in the upstream region, which behaves like a plasma bullet with a velocity of about 0.8×105 m/s. It indicates that streamer breakdown mechanism is involved in the upstream discharge. Electron density in the order of 1015 cm-3 is also obtained by Stark broadening. Electron excited temperature can be calculated by Boltzmann plot. The results show that the excited electron temperature in the upstream region increases with the increasing of the peak value of the applied voltage.