It is desirable to achieve the satisfactory transient and steady-state performance of single-phase T-type three-level converters in the active front-end application of the drive system. To this end, a fixed-time extended state observer (FTESO)-based improved super-twisting sliding mode (STSM) control strategy is proposed in this paper. Firstly, an improved STSM control is proposed by introducing the extra exponential function term into the conventional STSM controller. The improved STSM outperforms the conventional one with a faster dynamic response and superior steady-state performance simultaneously. Additionally, an FTESO is adopted in the voltage regulation loop to achieve a faster convergence speed and higher estimation accuracy as compared to the conventional ESO. This helps to strengthen the disturbance rejection ability of the converter against unknown external disturbances on the load. The external disturbance estimated by FTESO is added to the improved STSM controller as a compensation, assisting in the disturbance rejection of the controller. The proposed control scheme distinguishes itself with improved DC-link voltage dynamic regulation performance, enhanced disturbance rejection ability, fast power tracking performance, and weakened chattering in the steady state. Finally, experiment results are provided to verify the effectiveness of the proposed approach.