The invention discloses a power transformer defect precedent citing diagnosis method applying oil chromatography monitoring data. Currently, transformer defect diagnosis is still a work demanding rich experience. Because of lack of an effective mathematical model, in an actual production process, the transformer defect diagnosis is mainly based on experience-based judgment of personnel for measuring, regulating and operating, and thus it is hard to make breakthroughs in both the diagnosis efficiency and accuracy. Aiming at the distribution characteristics of oil chromatography characteristic gas data, a normalization hypercube mapping method is put forward, and the oil chromatography data is mapped to a hypercube spatial area which can be directly applied; meanwhile, a precedent citing similarity algorithm and a diagnosis result judgment method which is based on weighting vote are put forward pertinently. According to the power transformer defect precedent citing diagnosis method applying the oil chromatography monitoring data, the power transformer defect diagnosis problem is solved, the analysis accuracy is improved, and the capacity of a power grid operation and maintenance department in processing the defects and faults of oil immersed type transformer equipment is promoted.