An apparatus for defect pre-warning of a power device, comprising: a measurement data processing unit (110, 610), configured to process more than one measurement data regarding more than one components of the power device based on predetermined severity criteria, to obtain more than one severities of more than one monitoring indexes related to more than one defects of the more than one components; an index weight determination unit (120, 620), configured to determine more than one weights of the more than one monitoring indexes with respect to the more than one defects, based on effectiveness of the more than one monitoring indexes for the more than one defects; a defect probability estimation unit (130, 630), configured to estimate a defect probability that at least one of the more than one components has each of at least one of the more than one defects, based on the more than one severities and the more than one weights; a defect confidence estimation unit (140, 640), configured to estimate a defect confidence that the at least one of the more than one components has each of the at least one of the more than one defects, based on the more than one weights and the effectiveness of the more than one monitoring indexes; and a defective component probability estimation unit (150, 650) configured to estimate a defective component probability that the at least one of the more than one components is a defective component based on the defect probability and the defect confidence; wherein each monitoring index is a parameter which is related to a defect, and which is determined from the measurement data; wherein each severity is a degree that a value of the respective monitoring index is close to a threshold that will result in a defect with a very high probability; wherein each effectiveness is a degree that an estimation result is close to the truth if the respective monitoring index is used to estimate the defect, and wherein effectiveness is determined in advance. a measurement data processing unit (110, 610), configured to process more than one measurement data regarding more than one components of the power device based on predetermined severity criteria, to obtain more than one severities of more than one monitoring indexes related to more than one defects of the more than one components; an index weight determination unit (120, 620), configured to determine more than one weights of the more than one monitoring indexes with respect to the more than one defects, based on effectiveness of the more than one monitoring indexes for the more than one defects; a defect probability estimation unit (130, 630), configured to estimate a defect probability that at least one of the more than one components has each of at least one of the more than one defects, based on the more than one severities and the more than one weights; a defect confidence estimation unit (140, 640), configured to estimate a defect confidence that the at least one of the more than one components has each of the at least one of the more than one defects, based on the more than one weights and the effectiveness of the more than one monitoring indexes; and a defective component probability estimation unit (150, 650) configured to estimate a defective component probability that the at least one of the more than one components is a defective component based on the defect probability and the defect confidence; wherein each monitoring index is a parameter which is related to a defect, and which is determined from the measurement data; wherein each severity is a degree that a value of the respective monitoring index is close to a threshold that will result in a defect with a very high probability; wherein each effectiveness is a degree that an estimation result is close to the truth if the respective monitoring index is used to estimate the defect, and wherein effectiveness is determined in advance.