The steadystate operation principle of Zsource threephase fourleg inverter was analyzed, and an improved threedimensional space vector pulse width modulation (3DSVPWM) strategy was proposed. By properly selecting the shootthrough leg, the shootthrough states were inserted into the traditional zerovector state on the premise that the active switching state and output voltage were not affected. At the same time, the coupling relationship between the shootthrough duty ratio and the modulation ratio was analyzed, and the maximum value of the shootthrough duty ratio was obtained. Then the corresponding maximum constant boost strategy was proposed. A simulation model of the Zsource threephase fourleg inverter was built in MATLAB/Simulink and simulated under balanced and unbalanced load conditions. The simulation results showed that the Zsource threephase fourleg inverter could not only achieve boost control under the proposed 3DSVPWM strategy, but also achieve good control results under unbalanced load conditions compared with the traditional Zsource threephase threeleg inverter.