Interharmoics are the signals whose frequencies are between two harmonics. Besides the usual characterics of the harmonics, interharmonics will influence the present harmonic compensating equipment, and then result in failure compensation. Therefore, precise detection of interharmonics is very important and significant. Complex modulation Zoom-FFT (ZFFT) based on complex analytical bandpass filter is introduced in the real-time detection of the power system. Contrasting to the traditional zoom spectrum algorithms, the proposed algorithm is more suitable in the intensive spectrum analysis. Detailed theoretical analysis and simulation results are presented. The effectiveness of this algorithm is verified.