In this paper, the “random test-motion adjustment” method was used for the adjustment of the triangulation net of the Xinfengjiang Reservoir earthquake area, China, and for the inversion of the focal parameters of the earthquake of magnitude 5.3. Retriangulation and leveling data of the earthquake area were thus processed. The estimated focal parameters of the earthquake are: strike, NE 62°; dip, SE at 59°; length of the fault, 4.7 km; depth of the upper boundary, 0.45 km; depth of the lower boundary, 4.5 km; dip-slip dislocation, 73.4 mm; strike-slip dislocation, 25.5 mm; seismic moment, Mo, 2.2 × 1024 dyne-cm; stress drop (Δσ), 12 bars; and the lower limit of the strain energy released (E) 1.2 × 1019 ergs. It was predominantly a normal fault. This paper concludes that the regional stress field provided the main force of the shock, whereas the stress field due to hydraulic pressure contributed the inducing force.