Abstract: A below-elbow prosthetic arm assembly includes a hand; an arm connected to the hand; a cylinder connected to the arm; and a string connected to the rear end of the cylinder. To start using the prosthetic arm, the user's amputated upper limb is enclosed by the cylinder and tied thereto with a string. The below-elbow prosthetic arm assembly further includes an expansion kit adapted to meet the user's needs in daily lives, learning and sports. The expansion kit is directly fitted to the hand. Function expansion is achieved without removing the hand. Therefore, the below-elbow prosthetic arm exhibits enhanced ease of use and minimizes the user's financial burdens which might otherwise arise from ordering customized prosthetic arms for use in different activities.