This article introduces a harmonic detection device based on ARM/ DSP. This device uses DSP technology to analysis the harmonic wave in the power system by using complex sequence fast Fourier algorithm, and realizes the power system harmonic on-line detection with the strong control function of ARM module. It overcomes the inconsistency of the time sequence in communicating between DSP and ARM, and solves the synchronous sampling problem of each phase voltage for data acquisition module. In the program it mainly solves the HPI interface of data communication problem between DSP and ARM. Practical applications show that the device can rapidly and accurately detect characterization of power quality, and realize the telecommunication of the power grid data. It can detcet data accurately and reach the level of practical applications.