A synergy of fine-tuning the carrier concentration and implementing scatter of heat-carrying phonons is vital to the thermoelectric study of narrow bandgap semiconductors. In this work, we adopted a synergistic charge-balanced compensation-doping approach derived from the interplay between heterovalent Sb dopants and Sn vacancies in narrow band gap semiconductor SnTe, a lead-free alternative to the classic thermoelectric material PbTe. Specifically, we designed a composition series of Sn1-xSb2x/3Te (0 ≤ x ≤ 0.20 in steps of 0.02), in which every three Sn2+ are substituted by two Sb3+ and one Sn vacancy. To the first order, such chemical composition does not contribute net charge carriers in the context of electron counting and also ensures that the composition of the primary phase is not altered by the formation of secondary phase Sb2Te3, which greatly simplifies the data analysis. Interestingly, we found that the measured carrier concentration gradually reduced with increasing Sb content and in turn increased the Seebeck coefficient. Meanwhile, the coexistence of Sb dopants and Sn vacancies effectively scattered heat-carrying phonons at elevated temperatures. Consequently, a peak ZT value of ~1.1 at 873K was achieved in the x = 0.12 sample. These results offer a new avenue for thermoelectric study of narrow bandgap semiconductors.