According to the tough field investigation that last more than two months in the target area, the authors find the Shuiquanzigou-Tianshankou fault that caused the 2003 earthquake reveals the distribution of this fault and the latest active information, that is, the Shuiquanzigou-Xishanwan-Tianshankou fault consists of Shuiquanzigou-Xishanwan section which cuts the stratum of Mid-Pleistocene, being active in Mid-Pleistocene derived from ESR dating of fault clay and Yangshulin-Tianshankou section based on the materials collection of geophysics and seismology related to this event and the interpretation of remote sensing images. Finally, the authors consider the section of Shuiquanzigou-Xishanwan as the seismogenic fault based on the synthetical analysis of the focal mechanism, the distribution of the seismic damage and aftershocks.