By using a tri-electrode dielectric barrier discharge device, a uniform discharge with fairly large volume is realized in the main discharge region in atmospheric pressure air. The characteristics of the main discharge are investigated by optical and electrical methods. Results show that the main discharge includes two discharge modes with changing the input power of the source, which are a plasma plume discharge and a plasma column discharge. The inception voltage decreases with the increase of the peak value of the applied voltage for plume discharge. It can be deduced that the pre-ionization of air in the main discharge region is important for the main discharge, which results from the UV emitted from the coaxial dielectric barrier discharge. Spatially resolved measurements are conducted on the plasma plume and the plasma column with photomultiplier tubes. It is found that the plasma plume behaves like a plasma bullet that corresponds to a fast moving layer of light emission zone. Comparatively, the discharges at different positions of the plasma column almost volley, which means that the plasma column is of continuous discharge. Through analyzing the optical spectrum emitted from the main discharge, the vibrational temperature and rotational temperature are calculated. Results indicate that both the vibrational temperature and the rotational temperature decrease with the increase of peak value of the applied voltage. These results are of significance for the industrial applications of dielectric barrier discharge.