A method for detecting a deterioration defect of a structural part using a structural unit includes: defining a first base structural unit and obtaining N first base modal parameters of the first base structural unit; detecting a respective one of M first subject structural units of a subject structural part by each of M first sensors to obtain N first actual modal parameters of each of the M first subject structural units; calculating N first subject modal parameters of each of the M first subject structural units according to the N first actual modal parameters of each of the M first subject structural units and M*N first predetermined ratios; and comparing the N first subject modal parameters of each of the M first subject structural units to the N first base modal parameters to determine whether each of the M first subject structural units has a deterioration defect.