For the variational mode decomposition (VMD) is susceptible to noise and the mode decomposition number (MDN) is difficult to be determined in harmonic detection, a VMD harmonic detection method based on Wavelet Packet Transform (WPT) denoising and MDN optimization is proposed. Firstly, WPT is used to reduce the noise of harmonic signal, then VMD is used to predecompose the denoised signal, and the optimal MDN is selected according to the instantaneous frequency mean of analytical component. Finally, VMD is used to decompose the denoised signal under the optimal NMD, and harmonic parameters are detected by the Hilbert transform. The simulation results show that method can effectively select MDN, reduce the impact of noise, and has good accuracy.