Mechanism of gravitational erosion in small watersheds with or without vegetation or check dam on the Loess Plateau was explored with the aid of FLAC^3D, a finite difference software, so as to reveal mechanisms of some major engineering measures (building of check dams) and biological measures (maintenance of vegetation) regulating gravitational erosion. By comparing watersheds with and without controlling measures in distribution of displacement field, stress field and plastic deformation zone, effects of check-dam and vegetation reducing gravitational erosion were explored. Results show that hat vegetation may reduce displacement of hill tops and mid/lower slopes by 9.8% and 11%, respectively, while a check dam could only reduce displacement of the dam site by 10%. Moreover, vegetation could reduce volume of the plastic zone by 46% while a check dam could do only by 11%. A check dam increased concave slopes, while vegetation or roots reinforcement improved surface soil mass stress, both being able to reduce stress concentration of the soil mass on the slope and volume of the plastic deformation zone, but unable to alter the yielding model dominated by shear failure in the small watershed. However, within a small-scale range of a small watershed, the effect of a check dam was better than vegetation in erosion reduction, whereas in a large-scale range, it was just the reverse. The findings of the research can be cited as scientific basis for rationalization of the disposition of soil and water conservation measures for a small watershed.