A cascaded conversion system and a voltage equalizing control method thereof are provided. The cascaded conversion system includes a plurality of conversion circuits connected in cascade. Each conversion circuit includes a DC-side capacitor, a switching unit and a control unit. The DC-side capacitors of the conversion circuits are electrically connected in series. In each conversion circuit, the switching unit is connected to the DC-side capacitor in parallel and includes a plurality of bridge arms. Each bridge arm includes a first switch and a second switch. The control unit controls the switches according to the voltage across the DC-side capacitor. The control unit controls the first and second switches to be turned on alternately. All the first switches are turned on and off simultaneously, and all the second switches are turned on and off simultaneously, thereby making the voltages across the DC-side capacitors of the conversion circuits equal.