Abstract: In order to optimally allot the component mass of serial mechanism, a mass allot method is proposed based on the 3-DOF system. Firstly, in the case of the stiffness of the system is invariant, a dynamic model is constructed to research on the influence of different mass distribution on the first order natural frequency. The influence rules of the block mass on the dynamic stiffness and the kinematic accuracy of end effector are then analyzed by sensitivity analysis and line fitting. Following, two type of mass optimally allot methods based on total mass upper limit of the system and the first order natural frequency lower limit of the system are presented. The results show that the mass of block closely related to the dynamic characteristics of the system, and the total mass of the system than the dynamic stiffness of the system proportional relationship. The mass ratio not only can be used the mass allot for the initial design stage of the serial mechanism and the lightweight for mechanism design later period, also as evaluation index of the serial mechanism mass distribution rationality and the dynamic characteristic.