The invention discloses an improvement method and system of the online monitoring data quality of gas dissolved in oil. The method comprises the following steps: S1, obtaining online monitoring data of the content of the gas dissolved in oil of a transformer; S2, removing abnormal values in the online monitoring data, and replacing the abnormal values with null values; S3, complementing the null values through a cubic spline interpolation method; S4, distinguishing the abnormal values in the online monitoring data through a 3 sigma rule, and replacing abnormal values with modal numbers; S5, performing smoothing process through a low pass filter to remove random noise and white noise in the online monitoring data; S6, outputting data subjected to data quality improvement in the S1 to S5. The system comprises an acquisition unit, a replacement unit, a supplementing unit, a smoothing unit and an output unit. Through data supplementing, singular value removal and data smoothing processingon the original online monitoring data, processed data can represent the variation trend of the content of the practical gas dissolved in oil.