Disclosed is an apparatus and method for estimating a parameter of a secondary battery. The apparatus includes a sensor unit configured to repeatedly measure a voltage of a secondary battery including a blended cathode material containing at least first and second cathode materials at time intervals when the secondary battery is in an operation state or in an idle state, and a control unit configured to receive the repeatedly measured plurality of voltages from the sensor unit, identify an inflection point in a voltage change profile corresponding to the plurality of voltages, and estimate a depth of discharge (DOD) of the secondary battery by using a voltage measured after the inflection point is identified as a reference voltage.