Rapid plasma vertical displacement balance control (PVDBC) by Tokamak magnet power supply (TMPS) is essential in controllable nuclear fusion power generation (CNFPG). Significant increase in TMPS capacity is not conducive to commercial application of CNFPG, so optimization design of TMPS capacity is necessary from economic cost perspective. PVDBC system is established by combining plasma control system (PCS) model, TMPS output current fast control model and PVDBC model. Influence of current prediction on TMPS capacity is analyzed. Constraints for optimizing TMPS capacity are provided. Influence of different plasma vertical displacement error (PVDE) on required TMPS capacity for PVDBC is analyzed. Simulation analysis show TMPS output current response speed is effectively accelerated by predictive control, and under a certain PVDE, PVDBC is realized with TMPS capacity optimization by predictive control. To reduce TMPS capacity, predictive control is applied to PCS and PVDBC is realized when small PVDE occurs, providing new approach for optimized joint design of PCS and TMPS.