In this paper, we present a low-damping active knee prosthesis (LDKP) with low noise and high backdrivability. The proposed prosthesis is driven by a motor and then decelerated by a four-stage synchronous belt. High backdrivability given by this structure accelerates the prosthetic response. A control system containing several sensors are embedded in the proposed prosthesis to recognize different modes and provide corresponding strategies. Preliminary experiments were carried out on a transfemoral amputee subject, demonstrating the features of low noise, high backdrivability and ability to reproduce natural walking gaits.