The invention discloses a method for selecting an optimal length of oil chromatogram online monitoring data, which belongs to the technical field of online monitoring and analysis of a transformer oilchromatogram. The method comprises the steps of obtaining oil chromatogram online monitoring data of a transformer to serve as an original oil chromatogram data set; reconstructing a phase space of the original oil chromatogram data set; calculating an optimal embedding dimension and an optimal time delay of the phase space of the oil chromatogram; according to the contractility of the phase space, analyzing a relationship between the optimal embedding dimension and the optimal length; according to the optimal length, obtaining an optimal oil chromatogram analysis data set from the original oil chromatogram data set; and finally outputting the optimal oil chromatogram analysis data set. The optimal analysis data set can be obtained from a large amount of the oil chromatogram online monitoring data, so that the analysis efficiency and the analysis accuracy are higher.