In reactive power compensation technology, when refers to the reactive current detection, the design of the low-pass filter is one of the key issues. For the reactive current detection methods, the testing process which relies on the theory of instantaneous reactive power was derived in this paper. Different IIR digital filters were designed by Matlab. This paper gave out a comparative study on their design ideas and their performances, and selected the appropriate low-pass filter by the comparison. This paper designed and developed the digital low-pass filter which is based on field programmable gate array (FPGA) in the Quartus II 6.0 design platform, and the experimental results verified the correctness and validity of the design method.