In this paper, a new portable digital instrument is developed to measure the frequency, rms voltage, rms current and harmonic of power grid. Three phase voltage and current are sensed with precise transformer and then processed by signal conditioning unit before sampling. Synchronous AC sampling technique is introduced to avoid leakage effect of FFT. Consequently, frequency estimation, rms measurement and harmonic estimation are achieved based on MSP430. The experiment results indicate that this analyzer provides accurate frequency estimates with error in the range of 0.001Hz and amplitude estimates with error in the range of 0.1%, which can meet the need of offline application.