The invention discloses a transformer fault diagnosis method based on a coupled hidden Markov model, which belongs to the technical field of electrical equipment state monitoring and fault diagnosis. The method adopts left and right hidden Markov chains, measured values of five gases in oil chromatography are input as observed values of one chain, corresponding ratio values of the five gases are input as observed values of the other chain, and then transformer fault diagnosis is carried out. The method provided by the invention retains the advantages of the hidden Markov model, is applicable to analysis of non-stationary signals with poor repeatability and can carry out fault diagnosis based on multichannel information.