This study presents the innovative design and fabrication of a 3D-printed prosthesis arm aimed at enhancing automated hand rehabilitation for targeting hemiplegic patients who cannot open and close their hands. By leveraging surface electromyography (EMG) signals, the prosthesis arm translates muscle contractions in the forearm into precise hand movements, facilitating more effective rehabilitation. A comprehensive analysis of existing prosthesis designs guided the creation of a new, improved prototype. Using a concept scoring evaluation matrix, the best concept design was selected, designed, and fabricated through additive manufacturing. The finished prosthesis improves grip stability by imitating natural finger movements with flexible materials and a tendon system made of elastic ropes and fishing lines. Experimental comparisons between the designed prosthesis arm and a human hand showed significant similarities in flexion angles, though further refinement is needed for measurement accuracy and tactile feedback. The result presents the designed prosthesis arm validity and can be used in automated control rehabilitation in the future. This research also highlights the potential of 3D printing and EMG technology in creating customizable, efficient prosthesis arms for automated rehabilitation, suggesting pathways for future improvements and broader applications.