Abstract: Using the arrival times from the aftershocks of 2017 Jiuzhaigou M_S7.0 （M_W6.5） earthquake, the P- and S-wave velocity structures in the source region and its adjacent area are obtained by means of the body wave tomography method. The results show that the aftershocks, characterized by NNW-striking, SW-trending and a high dip angle, are mainly concentrated on one side of the low-velocity anomalies at the intersection of high- and low-velocity anomalies. The P- and S-wave velocity structures reveal the differences of crustal property on both sides of the seismogenic fault: the hanging-wall is a high-velocity zone with strong rigidity while the footwall is a low-velocity zone with weak rigidity. According to the distribution of aftershocks and the seismic velocity structure, it can be inferred ultimately that the 2017 Jiu-zhaigou M_S7.0 earthquake occurred on the lower interface of the upper crust, following with the rupture propagated in a dramatically deformed area in the upper crust and terminated near the depth of 25 km, and the seismogenic fault has the property of low-velocity hanging-wall and high-velocity footwall.