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The traditional detection methods are easily affected by noise. It is difficult to extract the useful signal from the power traveling wave which is submerged in a lot of noise. A method using digital lock-in amplifier is proposed for detecting the power traveling wave signal. The basic principles of the lock-in amplifier are studied. Virtual instrument language LabVIEW is used to design the digital lock-in amplifier. It is able to extract the corresponding frequency components of the power traveling wave signal. As the front panel and the flow sheet of the digital lock-in amplifier are presented, the design of each module is introduced in detail. The experimental research on detecting signals containing noise and not containing noise are carried out, respectively. The results of experiment show that the presented digital lock-in amplifier can accurately extract the corresponding frequency components of the power traveling wave signal. Moreover, with the increase of the noise amplitude, the root mean square error of the output signals varies little. The presented digital lock-in amplifier is able to cancel the effect of noise. Therefore, it is a powerful tool to detect and extract the power traveling wave signal.