Multimaterial soft robots, composing of integrated soft actuators and a relatively harder body, show great potential to exert higher payloads and support their own weights. This paper proposed a systemic framework to automatically design and fabricate this kind of robots. The multimaterial design problem is mathematically modeled under the framework of topology optimization in which the structure and material distribution are obtained simultaneously. Herein, a multimaterial pneumatic soft finger, modeled as a compliant mechanism, is optimized to achieve its maximal bending deflection and further customized to practical applications on grippers, rehabilitation and an artificial hand. These optimized multimaterial soft fingers are fabricated by combining molding and 3D printing technique. Experimental results show that the soft gripper can manipulate a large variety of objects with different shapes (from M4 screws to complicated sunglasses) and weights (up to 168g), the rehabilitation finger can facilitate human safely in two modes, and the artificial hand can perform various gestures. This work represents an important step toward the realm of high-performance soft robots.