Because the current 3-UPU redundant actuation parallel mechanism has singular defects, the redundant branch chain selection method has low reliability and uneven distribution of inflexibility. A new redundant branch chain optimization method of 3-UPU redundant actuation parallel mechanism was proposed. The 3-UPU redundant actuation parallel mechanism and the requirement of freedom degree were designed. The pose parameters of the moving platform were regarded as variables. The force matrix and the torque matrix of 3-UPU redundant actuation parallel mechanism were obtained. The input of 3-UPU redundant actuation parallel mechanism was obtained. The driving screw parameter and the driving screw of non redundant branch chain were obtained by the position equation, so as to improve the reliability. The parameters of redundant actuation spiral were calculated to get the related information of condition number mechanism, and then the force matrix and torque matrix of stiffness matrix were determined. Finally, the dexterity and stiffness were used to obtain the redundant branch chain which has the optimal condition number and stiffness, so as to solve the problem of uneven distribution of stiffness. Experimental result shows that the proposed method has high reliability and uniform stiffness distribution.