In recent years the power harmonic pollution is increasingly serious, precise and realtime harmonic detection can provide a important reference for the harmonic control. TMS320F2812 DSP is a chip with powerful data processing capability and control function. In this paper, using the DSP chip as the core controller and making full use of the chip resources, a power harmonic detection system with high precision is designed. The ADC calibration based on hardware&software aspects and synchronized sampling based on software phaselocked of Capture unit(CAP) improve the precision of the system. Besides, This paper presents the key hardware circuits and software codes. The experimental results show that this system can realizes realtime accurate detection, as well as synchronized display and control, to the harmonic amplitude and phase.