For the voltage sag phenomenon of electrical system, and to be able to diagnose the situation promptly voltage sag, we propose a new detection algorithm: RV algorithm. In this study, TMS320C40 board, filter and some other softwares are used to collect some voltage signals which include normal voltage and voltage sag caused by different reasons, and the rectified voltage (RV) algorithm is applied to calculate, simulate and do some other treatment for a variety of voltage signals according to the voltage waveform and the corresponding parameters. Experimental results show that this algorithm can not only effectively detect the voltage sag conditions within a very short period of time, but also can save a lot of storage space and calculation.