Online testing not only realizes the real-time characteristic data, but also analyzes the data and forecasts operation status. But, in the power system, influence of harmonic and attenuation component will greatly increase error. In order to solve the greater error caused by decaying DC component in full-wave Fourier algorithm, an improved algorithm is proposed. First, characteristic parameters are calculated in addition to periodic component by adding two sampling points. Then, we use Fourier transform twice so that we can get the amplitudes and phases of fundamental and harmonic. Through a simulation by LabVIEW, it is proved that this method has higher accuracy, faster calculation speed and is superior to the traditional algorithm, especially to the fundamental component. Using LabVIEW is conducive to extended function.