Abstract: The harmonic reducer used in aerospace requires high transmission accuracy，and the error caused by the misalignment problem of the reducer cannot be ignored.Therefore，the fault feature extraction method of the misalignment problem of the harmonic reducer has become the focus of the research.A new method termed optimal variational mode decomposition（VMD）was proposed to extract harmonic reducer misalignment fault features.Firstly，since envelope entropy is very sensitive to fault signal features，it is used as a fitness function，which is an objective function for the whale optimization algorithm（WOA）.Secondly，WOA has numerous merits such as simple operation，fewer adjustment parameters and a strong ability for jumping out of local optimum，and is applied to the optimization of VMD.Finally，intrinsic mode function components were processed through a Teager energy operator.The suggested approach was employed to analyze the experimental signal collected from harmonic reducer.The comparison results show that the advanced method is more effective and demonstrates superiority over empirical mode decomposition，local mean decomposition and wavelet packet decomposition.

Key words: harmonic reducer, optimal variational mode decomposition, misalignment fault feature extraction, envelope entropy, whale optimization algorithm