Ultra-high-power and high-resolution amplifiers are unitized in magnetic resonance imaging to generate high-fidelity gradient fields, which directly impacts imaging quality. A state space control method, in particular an order-reduced state space controller, are developed in this paper, for sake of developing a power amplifier with MVA power level and a resolution at PPM level. Finally, a 900 A / 1400 V ultra-high-power amplifier are used to demonstrate and prove the effectiveness of the developed method.