As a novel three-level topology, the three-phase VIENNA rectifier is widely applied in high-power rectification scenarios owing to its characteristics such as high power density, low voltage stress and unity power factor operation. Aimed at the double-frequency fluctuation generated by DC-link voltage when the grid is unbalanced, an improved unbalanced control strategy is designed under the two-phase stationary reference frame. First, the separation between the positive- and negative-sequence components of grid voltage is realized by means of the decoupled double synchronous reference frame PLL. Then based on the positive- and negative-sequence dual-current loop independent control structure, sliding mode control (SMC) is introduced into the current controller. By selecting an appropriate sliding surface according to the set tracking error and the sliding mode approach law, the DC-link voltage ripple is suppressed and the input current harmonic component is minimized. Finally, a simulation model of VIENNA rectifier control system under unbalanced grid conditions is built in Matlab/Simulink, which verifies the correctness and feasibility of the proposed control strategy.