For the low accuracy and positioning difficulty to detect inter-harmonics, a new method is discussed in this paper. It calculates the relative wavelet packet energy based on wavelet packet. The spectral features are obtained by using FFT algorithm. According to its relative wavelet packet energy and its spectral features, it can determine the frequency components of the detected power signal. The transformed signal is reconstructed using wavelet packets. The amplitudes and its frequencies of inter-harmonics of the reconstructed signal are calculated using apFFT algorithm. The proposed algorithm is simulated. The simulation results are compared with those of literatures. It can be seen that the proposed algorithm is verified, and the proposed algorithm can detect inter-harmonics of a given signal much more accurately.