The plasmas generated by dielectric barrier discharge in atmospheric pressure air have wide application prospect in industry. In order to study generation condition and mechanism, the dielectric barrier uniform discharge in atmospheric pressure air has been studied experimentally with a micro-gap discharge device. Results of electrical characteristics indicate that it exist several current pulses with short width in half period of the applied voltage at a low voltage, a large number of micro-discharge filaments are observed. The discharge power increases with increasing peak value of applied voltage, the micro-discharge filaments increase meanwhile. When the peak of applied voltage reaches to 9.2 kV, only a discharge hump with a width of about 5.5 μs appears in a half period of the applied voltage, micro-discharge filaments cannot be discerned. The uniform discharge has been obtained finally as the micro-discharge filaments extend and superimpose randomly. The emission spectrum of dielectric barrier discharge scanning from 330 to 420 nm is collected. It is found that the intensity of 337.1 nm is stronger than that of 391.4 nm. If the intensity of 337.1 nm is considered as the datum reference, the intensity of 391.4 nm shows the magnitude of electron average energy. The molecule internal energy is evaluated by vibration temperature. The electron average energy and molecule internal energy have been investigated by optical emission spectra. It is found that both of them decrease with increasing the applied voltage. Results indicate that it is not easy to form filamentary discharge when the electrical energy is lower. The average electron energy of uniform discharge is lower than that of the filamentary discharge. These results are of great significance to the application of dielectric barrier uniform discharge obtained in air at atmospheric pressure.