In this work, Sr_0.7Bi_0.2TiO₃ (SBT) was doped into BF-BT to form a solid solution with relaxor ferroelectric characteristics. Constricted P-E loops were observed due to the field-induced phase transition and a significant reduction of grain size was found in the SBT-doped ceramics. Specially, 15%-SBT doped ceramics (15SBT) possessed the maximum recoverable energy storage (W_rec) of 2.01 J/cm³ and efficiency of 75%. Finite element method was used to analyze the local electrostatic behavior of ceramics under the electric field when the second phase exists. The simulation results indicate that the distributed second phase with moderate fraction has a positive effect on the enhancement of breakdown strength (BDS). More importantly, a viscous polymer process (VPP) route was employed to realize preparation optimization of the above composition. Much denser structure and higher BDS was successfully obtained in the 15SBT ceramic by VPP. The BDS of 15SBT ceramic by VPP has been dramatically increased from 180 to 330 kV/cm, and the W_rec has reached 4.95 J/cm³. Further charge-discharge tests of 15SBT ceramic by VPP also owns a high discharge energy of 2.36 J/cm³. The 15SBT ceramics prepared by VPP can be a potential candidate for high energy storage capacitors. Furthermore, the strategy raised in this study has been proved to be an effective way to achieve excellent energy storage capacity in lead-free ceramics.