Search for Extra Terrestrial Intelligence (SETI) is an important subdomains in radio astronomy. In order to obtain as much observation time as possible, SETI adopts the Commensal Survey which does not occupy the telescope's time alone, searching for SETI signals while the telescope performs other observation tasks. In this paper, the concept of SETI commensal observation and the overall framework of SETI backend are introduce, and the main strategies of SETI commensal observation are analyzed. The real-time data receiving system SERENDIP ia analyzed and explained. At the same time, radio frequency interference (RFI) removal and candidate target extraction methods are analyzed. The validity of SETI backend is verified by processing FAST's 5-hour drift scan data. Finally, the future development trend of SETI is prospected. The high sensitivity of the 500 meter aperture spherical telescope (FAST) makes this project not only an effective verification of other telescope observation data, but also increases the possibility of detecting weak extraterrestrial signals.