Dielectric barrier discharge (DBD), as a popular low-temperature plasma source, has drawn significant attention due to its extensive application fields including surface modification, material synthesis, and sterilization. The DBD has presented different modes under varying experimental conditions. In order to address the formation mechanisms of the different modes, a two-dimensional axis-symmetric fluid model is employed to simulate the characteristics of DBD in atmospheric pressure argon. The results indicate that the DBD undergoes a scenario from a discretely-filamentary mode, a diffuse mode, a complementarily-filamentary mode, to a columnar mode with the increase of voltage amplitude (V_a) or discharge power (P_dis). The waveforms of applied voltage and discharge current indicate that for each discharge mode, the discharge current waveforms are always symmetrical for positive and negative discharge half-cycles. The discharge current exhibits single-pulse characteristics per half-cycle with low V_a (or P_dis), and turns into double-pulse, triple-pulse, or multi-pulse characteristics per half-cycle as V_a (or P_dis) increases. Spatial-temporal evolutions of electron density and electric field reveal that residual electrons play an important role in the discharge mode. Electric field (E) is mainly composed of its axial component, and its radial component only appears at the edge of the electrode in the diffuse mode. In the complementarily-filamentary mode, the locations of the strong-MDs and those of the weak-MDs alternately appear in the consecutive half-cycles. The strong-MD channels are stationary at fixed locations in the consecutive half-cycles for the columnar mode. In addition, the channel diameter of residual electrons in the columnar mode is larger than that in the filamentary mode. Moreover, as V_a (or P_dis) increases, the generation rate of Ar* increases, while the energy efficiency of the discharge decreases. These results are of great significance for in-depth understanding discharge mode and improving DBD performance.