The invention discloses a frequency converter device and an over-current protection method thereof. The frequency converter device is provided with a frequency conversion part, a phase current detection part and an over-current judgment part. All bridge arms of the frequency conversion part are respectively connected with all phase lines. As far as all the bridge arms, the bridge arms are respectively divided into an upper bridge arm and a lower bridge arm by connection nodes respectively connected with the phase lines; each upper bridge arm and each lower bridge arm are respectively provided with a switching element for switching on/off the corresponding bridge arm; the phase current detection part comprises a plurality of bridge arm resistors and a current detection part; the bridge arm resistors are respectively arranged on respective lower bridge arms of all the bridge arms; the current detection part is used for detecting respective voltages of both ends of each bridge arm resistor and outputting all phase current; the over-current judgment part comprises a prospective current operation part and a comparing part; the prospective current operation part is used for calculating a prospective current value generated in the later switching period by taking the phase current of the current switching period received from the phase current detection part as the reference; the comparing part is used for comparing the prospective current value with an over-current reference value; and if the prospective current value is greater than the over-current reference value, an over-current protection circuit is enabled to actuate.