This paper presents a finite-state control strategy for a robotic transtibial prosthesis to realize level ground walking with controlled damping behaviors in the ankle joint. We use a motor-driven method to generate braking torque to realize nonlinear damping behaviors. The controller performs damper control for the stance phase and angle control for the swing phase. We design and construct a robotic transtibial prosthesis prototype to evaluate the effectiveness of the proposed control strategy. One transtibial amputee subject participated in the experiments. By using the proposed finite-state control method, the robotic prosthesis can successfully mimick the behaviors of normal limb. The damping control strategy enabled the ankle to dorsiflex smoothly.