Complex structures with versatile chemistry provide considerable chemical tunability of the transport properties. Good thermoelectric materials are generally extrinsically doped semiconductors with optimal carrier concentrations, while charged intrinsic defects (e.g., vacancies, interstitials) can also adjust the carriers, even in the compounds with no apparent deviation from a stoichiometric nominal composition. Here we report that in Zintl compounds Mg3+xSb1.5Bi0.5, the carrier concentration can be tuned from p-type to n-type by simply altering the initial Mg concentration. The spherical-aberration-corrected (CS-corrected) high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDX) mapping analysis show that the excess Mg would form a separate Mg-rich phase after Mg vacancies have been essentially compensated. Additionally, a slight Te doping at Bi site on Mg3.025Sb1.5Bi0.5 has enabled good n-type thermoelectric properties, which is comparable to the Te-doped Mg-rich sample. The actual final composition of Mg3.025Sb1.5Bi0.5 analyzed by EPMA is also close to the stoichiometry Mg3Sb1.5Bi0.5, answering the open question whether excess Mg is prerequisite to realize exceptionally high n-type thermoelectric performance by different sample preparation methods. The motivation for this work is first to understand the important role of vacancy and then to guide for discovering more promising n-type Zintl thermoelectric materials.