A novel ferroelectric system, 0.2Pb(Mg1/3Nb2/3)O3–0.8Pb(Sn0.46Ti0.54)O3 with MnO2 addition (PMNST-Mn), was prepared. The dielectric, ferroelectric and piezoelectric properties were investigated. The results demonstrated that the addition of MnO2 suppressed the dielectric relaxor behavior of PMNST. With the increase of MnO2 addition, the diffuse phase transition (DPT) behavior weakened gradually. The addition of MnO2 contributed to the decrease of the dielectric loss (tanδ) and the enhancement of ferroelectric polarization. The optimum ferroelectric and piezoelectric properties were obtained when the addition of MnO2 is 0.75 mol%, and the remnant polarization (Pr) and mechanical quality factor (Qm) were about 25% and 60% higher than those of PMNST, respectively. It was suggested that the formation of oxygen vacancies made the important contribution to suppressing relaxor behavior and improving the electrical properties of PMNST due to the substitution of Mn for B-site. This work provided a practicable strategy to tune electrical properties of ferroelectrics.