This work investigated how the annealing process affects the ferroelectric properties of thin films of Sr0.8 Bi2.5 Ta1.2 Nb0.9 O9+x (SBTN) on Ir/SiO2/Si substrates prepared by two-target off-axis rf magnetron sputtering at various O2/(Ar+O2) mixing ratios (OMR) with a substrate temperature of 570 °C. Experimental results indicated that the annealing could effectively result in a large remanent polarization. The remanent polarization, dielectric constant and leakage current of 598 °C post-annealing SBTN thin films increased with an increase in the OMR and reached a maximum value at 40% OMR. In addition, the results obtained from the dielectric constant and the leakage current were interpreted in terms of polarization effect and loss theory. The 400-nm thick 40% OMR SBTN films with 598 °C post-annealing exhibited good surface morphology and had a dielectric constant of 752, a loss tangent of 0.035 at 100 kHz, a leakage current density of 6×10-6 A/cm2 at an electric field of 50 kV/cm with a delay time of 30 s, a remanent polarization (2Pr) of 40 μC/cm2, a coercive field (2Ec) of 77 kV/cm at an applied voltage of 3 V, and a measured value of Qsw of 20 μC/cm2. According to studies on the 10-year lifetime of time-dependent dielectric breakdown (TDDB), high OMR samples have a longer lifetime than the other lower OMR samples. The SBTN films demonstrated fatigue free characteristics up to 1011 switching cycles under a 3-V bipolar 1 MHz square wave. Moreover, the polarization of the films decreases slightly (less 0.5% per decade) with retention time up to 240 min.