In this paper, a carrier-based modulation algorithm is proposed for the traditional three-dimensional space vector modulation algorithm with the disadvantages of complicated computation process and large computation volume, taking the three-level four-bridge arm inverter as the object of research. This algorithm generates modulating waveforms by superimposing the offset voltage to the output voltage of the inverter, and compares the generated modulating waveforms with the triangular carrier waveforms to produce the corresponding switching signals, so as to realize the output of three-phase balanced voltages under the condition of unbalanced loads, and the digitalization is easy to realize. The computation process is simple, small and easy to digitize. Simulations and experiments verify the feasibility and effectiveness of the proposed algorithm.