The invention discloses a high-voltage frequency-conversion speed-regulation system and a control method thereof. The system includes a phase-shifting transformer, a frequency converter which includes 3n identical power units, optical-fiber interfaces and a control unit, wherein n is larger than or equal to 2 and is an integer. Every n power units are connected serially and sequentially into one phase and the power units are connected with a motor after being connected in a three-phase star-type manner. The phase-shifting transformer includes 3n secondary windings. Each secondary winding is connected with a power unit. The control unit is connected with the different power units through the optical-fiber interfaces. The method includes: an initializing step; an interrupt-vector-table updating step; a watchdog starting step; and a PWM interrupt repeating step. Through a multi-level PWM control method and realizes, the high-voltage frequency-conversion speed-regulation system and the control method thereof provide sine-wave current output and realize high-quality control of the motor. The control method enables the high-voltage frequency-conversion speed-regulation system to operate in an efficient, accurate and orderly manner so that high efficiency, high power factor and high reliability are realized.