Abstract: In this paper, the triangulation net adjustment in the Xinfengjiang reservoir area and the method of random try-motion adjustment used to get the fault parameters by inversion of the results of adjustment are systematically described. In treating the triangulation data before and after the magnitude 5.3 earthquake of the Xinfengjiang reservoir area it has been further made clear the applicability of the method.Through inversion, the parameters of the earthquake fault are: strike N62E, dipping to the southeast at an angle of 59, the length of the fault, 4.7 km, depth of its upper boundary, 0.45 km, lower boundary, 4.5 km, dip slip dislocation, 73.4 mm, strike slip dislocation, 25.5 mm. It is a normal right-lateral fault, seismic moment m0=2.01024 dyne-cm, stress drop =12 bars, strain energy release E=1.21019 ergs.It is also discussed the force system causing the earthquakes of the Xinfengjiang reservoir area based on the results thus obtained and the data of focal mechanisms and of tectonic structures. The conclusion is that the regional stress field constitutes the main cause, while the water pressure provides the triggering effect of the earthquakes.