Dielectric barrier discharge (DBD) can produce non-equilibrium plasma at atmospheric pressure, and it has become a hot point in recent years. For the DBD excited by pulsed or alternated currents, the effects of the loading performance of power supply, the matching between supply and discharge reactor and the discharge phenomena on its discharge are interesting issues. The studies of these issues are of great importance for understanding the DBD processes and improving the power supply efficiency. In this paper, the Lissajous figures of a DBD reactor with coaxial electrode configuration are measured. The loading performance of the DBD reactor and the dependences of excitation voltage and air flow rate on the dielectric layer equivalent capacitance are studied in atmospheric air. According to the experimental data and circuit modeling analysis, it is proved that the dielectric layer capacitance decreases with the increase of air flow rate, but increases with the increase of excitation voltage. The amplitude-frequency performance of the reactor reveals significant RLC circuit resonance. The resonance frequency of the reactor has the same behavior as its dielectric layer capacitance. Therefore it shows that the dielectric layer capacitance is the main factor for the resonance frequency evolution. A possible mechanism responsible for the dielectric layer capacitance is also presented.