The present invention relates to a ballast water treatment system. The ballast water treatment system comprises: a filter and electrolysis unit for receiving ballast water from the outside, and filtering and electrolyzing the ballast water; a first sensor unit for measuring seawater characteristics of the ballast water passing through the filter and electrolysis unit; a control unit for determining a contamination level of the ballast water according to the measured seawater characteristic values so as to generate control signals for determining the strength of electrolysis and the input amount of chlorine dioxide; a chlorine dioxide generation unit for generating the chlorine dioxide; a UV/TiO2 system which includes a TiO2-coated plate, generates an OH radical through an interaction between ultraviolet rays and TiO2 so as to enable the OH radical to sterilize residual microorganisms included in the ballast water passing through the filter and electrolysis unit; an automatic neutralization treatment device which includes a neutralizing agent, receives a neutralizing agent discharge signal from the control unit, and neutralizes the ballast water passing through the filter and the electrolysis unit; and a second sensor unit for measuring a TRO concentration of the ballast water neutralized in the automatic neutralization treatment device and transferring the measurement result to the control unit, wherein the control unit generates a control signal for controlling an amount of the neutralizing agent discharged from the automatic neutralization treatment device according to the measurement result received from the second sensor unit. Thus, the sterilization efficiency for the ballast water satisfies the USCG standards during the ballasting step for the ballast water.