The invention discloses a soft charging device for a high-voltage frequency converter and a control method thereof. The soft charging device comprises a high-power charging resistor R, contactors KM1 and KM2 and a vacuum breaker K1 at a high-voltage alternating current input side. The method comprises the following steps of: firstly charging an auxiliary winding by virtue of the soft charging resistor, so as to charge each unit slowly; then after charging is finished, disconnecting the contactor at the auxiliary winding side; and finally performing switching-on operation on the vacuum breaker at the high-voltage input side. By utilizing the device and the method, less equipment is used; the operation is simple; the charging excitation surge current of the transformer is reduced while the unit soft charging device is simplified; and the application value is good.