The HeI photoelectron spectroscopy is used not only in reseaching steady molecule but also in reseaching transient species. The system consists of a A/D card and a software to make the card to work. The system converts the analog signal (from the detector) to digital signal and processes the data in the computer. The method of designing Butterworth digital low pass filter is inteoduced in this paper. The digital low pass filter in the system improves signal-to-noise ratio.