Abstract: Coronal Mass Ejection (CME) is the process that huge amount of foamy gas which carries the magnetic field lines, is projected from the sun in a few hours. CME releases high levels of radiation and a large number of charged particles which will cause great disturbances to the Sun-Earth magnetic field when they travel to the vicinity of the Earth. CME is the driven source of the geomagnetic storms and moderate solar energetic particle events. The change of CME propagation direction determines whether a CME could arrive on Earth. Therefore, to study the deflection of CME could offer a better understanding about its affections to space weather, which is significant to predict space weather events. This paper analyzes the deflection data of CME observed by STEREO satellite on the 8 October 2007. It uses the Global Linear Force-Free Field model to extrapolate magnetic field and analyzes the relationship between the distribution of the background magnetic energy density and the deflection of CME. The trajectory of CME is calculated. The result shows that the theoretical trajectory of CME fit the actual observations most closely when the force free parameter α=0.15.