With the development of agricultural mechanization in China hilly region, how to allocate the field road network in sloping land reasonably has been one of the bottlenecks which restricted the development. The object of the previous road network density design was mostly aiming at the highway, whereas seldom involves the status of the off-rade road such as field road. A theoretical method with mathematic model has been established to determine the field road network density of sloping farmland in small watershed, and the solution of the model has been obtained according to the stability theory of differential equations. Using the Xiaziling small watershed in Zhong County as an example, the field road network density with the highest labor productivity and lowest construction cost has been given. The results can be helpful for the design of the field road networks in the small watershed of Three Gorges Reservoir regeon