Multi-material soft robots, composing of integrated soft actuators and a relatively harder body, show great potential to exert higher payloads and self-support. This work proposes a systemic framework to automatically design and fabricate this kind of robots. The multi-material structural design problem is mathematically modeled under the framework of topology optimization in which structure and material distribution are designed simultaneously. A multi-material pneumatic soft finger, modeled as a compliant mechanism, is designed to maximize its bending deflection and further customized to practical applications on soft gripper, rehabilitation and artificial hand. These optimized multi-material fingers are fabricated by combining molding and 3D printing technique. Experimental result shows that the soft gripper is able to manipulate objects a large variety of objects with different shapes (from Ø3 skews to complicate sunglasses) and weights (up to 168g), the rehabilitation finger can facilitate human safely in two modes, the artificial hand owes the potential to deliver message. This work represents an important step toward a realm of high-performance multi-material robots.