To improve the reliability and accuracy of digital sampling, the problem of white noise has become the research focus in the industrial and academic circles. First of all, the output waves related to merging unit of digital input are given, while the component of white noise is analyzed, together with a specific application of smart substation. Additionally, the relative digital filter is designed, considering the processing effect, stability and time delay, while the parameters and processing results are given. Finally, the influence of transient characteristics is researched and estimated, combined with the indexes of decay time constant and maximum peak instantaneous error. The results show that the digital filter could decrease more than half of the white noise, improve the sampling waveform, and would not influence the transient characteristics.