The invention discloses a frequency conversion speeder for a big-power motor with multiple bridge arms. The frequency conversion speeder comprises a power transformer, a rectifier circuit, a filter circuit and an inverter circuit; the secondary side of the power transformer is connected with the rectifier circuit; the filter circuit and the inverter circuit are connected in parallel between the positive generatrix(P+) and the negative generatrix(P-) of the rectifier circuit. The filter circuit comprises a filter capacitor and the inverter circuit comprises n(n is a natural number more than 3) bridge arms, each composed of two controllable pieces which are connected in series; voltage output is connected between the two controllable pieces which are connected in series. The frequency conversion speeder can realize bridge arm division according the volume of the power, thus solving the problems in speed regulation of a multi-phase motor.