The reactive power with high precision is more and more important, so several reactive power algorithms are compared systematically and comprehensively. This paper firstly describes reactive power defined by Budeanu, and investigates several algorithms based on the definition. In order to compare advantages and drawbacks of the algorithms, the fundamental theory and computational accuracy of the algorithms, including fundamental phase shift 90 degree, differential, FFT, Hilbert digital filter, are analyzed. At the same time, according to hardware platform performance, the algorithms are improved; furthermore the methods of software implementation are investigated. Finally, comparison tests of the algorithms are implemented at high accuracy three phase meter based on DSP. The analysis data shows that, under sinusoidal and non-sinusoidal conditions, simulation and experimental results perfectly coincide. The study is instructive for optimizing the design of reactive power algorithms and for electric energy metering.