Aiming at the problem of fault diagnosis of analog circuits with tolerance, a subband polymorphic spectra (SPS) approach of fault feature extraction is proposed in this paper, which is based on empirical mode decomposition (EMD). Firstly, the secondorder Volterra kernel sequences of the circuit under test are calculated. Then the Volterra sequences are decomposed based on EMD and the intrinsic mode functions (IMFs) are obtained. By calculating cepstrum (CS) and Hiltert spectrum (HS) from IMFs, the polymorphic digital fault features in timefrequency domain are extracted, the soft and nonlinear faults in analog circuits with tolerance are isolated and the fault diagnosis of analog circuit is achieved. The experiments results show that the proposed method in the paper can solve the fault aliasing problem effectively and improve the capability to locate and isolate fault components.