Disclosed is an apparatus and method for estimating a parameter of a secondary battery. The apparatus includes a sensor configured to measure an open circuit voltage (OCVm) of the secondary battery at a first depth of discharge (DOD′), the open circuit voltage (OCVm) corresponding to a difference between a predetermined original cathode voltage (Vc,o) at the first depth of discharge (DOD′) and an anode voltage (Va), and a control unit configured to calculate an anode voltage (Va) from the original cathode voltage (Vc,o) and the open circuit voltage (OCVm), determining that the original anode voltage (Va,o) is identical to the anode voltage (Va), determining a second depth of discharge (DOD″) corresponding to the original anode voltage (Va,o), and determining capacity degradation (Δcapa) of the secondary battery according to a difference between the first depth of discharge (DOD′) and the second depth of discharge (DOD″).