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This paper presents a two-dimensional direct numerical simulation (DNS) of dielectric fluid flow subjected to unipolar injection under an alternating current (AC) electric field. The effect of frequency f of pulsed direct current (PDC) and AC on the transient evolution of electroconvection and their subcritical bifurcations are investigated for the first time. Electroconvection under PDC or AC tends to exhibit oscillating flow due to the periodic boundary condition of electric potential and charge density compared with the direct current (DC) case. The results demonstrate that the linear stability criterion Tc decreases as the frequency increases under a PDC field, while the nonlinear stability criterion Tf is hardly affected. Under the AC field, a critical frequency fc = 0.0316 is found, which separates electroconvection into two typical flow regimes—periodic flow regime (f < fc) and inhibited flow regime (f ≥ fc)—depending on whether free charges can reach the collector electrode before electric field inversion. These mechanisms of electroconvection under PDC/AC field offer possibilities in the field of flow control.