Aiming at the problem of signal detection in low signaltonoise ratio in power line communication, the paper takes the most common modulation signal in power line communication— OFDM signal as detection object, and analyzes the favorable timefrequency analysis characteristics of wavelet packet transform. In the power line noise channel, according to the different wavelet packet decomposition characteristics of OFDM signal with the other single carrier signal, a detection method of OFDM signal based on wavelet packet transform is proposed by taking maximum value of wavelet packet decomposition coefficient norm two as detection characteristic quantity. The simulation results got by MATLAB show that this detection method can detect the OFDM signal in low signaltonoise ratio, and reflect advantages of the wavelet packet transform applying to the power line communication.