Aiming at the large fluctuation for chains length of two eccentric sprockets transmission mechanism at work, an eccentric tensional sprocket was set. The chains were always in tension through the optimized revolving center, eccentricity and initiatory phasic angle of the eccentric tensional sprocket. By kinematics analysis with differential equation, the relation equation of angle displacement between the main sprockets wheel and subordinate sprockets was obtained. Then, the optimization calculation for the position of rotary center, initiatory phasic angle and eccentricity of the tensional sprocket was accomplished, with a view of the minimal degree of tightness fluctuate of chain. An optimizing example was present. Furthermore, the consideration for the non-continuity of chain length was taken into in optimization. 