Interior assembling inside the cabin of an aircraft requires assembling robot to be light-weight and able to carry heavy payload. This paper proposed a hybrid robot and carried out its optimal design and experiments. The robot consists of a 1T2R parallel module and a 2T serial module. In the parallel module, the first limb is composed of a slider crank mechanism and a RS link. The other two limbs are PRS limbs. Herein, R, S, P are revolute, spherical, and actuated prismatic joints. Optimization of the robot concerns motion/force transmissibility, total mass, and stiffness. Hence, kinematic, stiffness, and mass modeling are implemented, and then the Pareto-based multi-objective optimization. Objective arrangements are discussed by concerning (1) the conflicting relation between mass and the minimal linear stiffness along z-axis and (2) the overall stiffness performance. After comparing six multi-objective optimizations, it is found that simultaneously regarding mass and minimal linear stiffness along z-axis as objectives is beneficial for obtaining large payload-to-mass ratio, moreover having overall stiffness as objectives would lower the values of motion/force transmissibility and payload-to-mass ratio. Finally, optimization model having motion/force transmissibility, total mass, and minimal linear stiffness along z-axis as objectives is selected. The optimal payload-to-mass ratio is up to 13.2837. The five degrees-of-freedom hybrid robot is machined and assembled. Experiments on the workspace, repeatability, and load carrying capacity confirm the performances of the designed robot.