Accurate phasor measurement can provide accurate dynamic data and fault information for power system. Electricity theft can be found by analyzing the phasors at different locations and at different times in the power grid. In this paper, a double spectral line symmetric interpolation FFT phase measurement algorithm is proposed, and a hardware design scheme of real-time phase measuring device for reverse power theft is presented. In this paper, the design principle of signal conditioning and acquisition module, data processing module and communication module is introduced in detail, and the simulation and measurement results are given at last.