There are millions of people who have lost a hand or both hands due to acquired amputation or congenital limb deficiencies. In order to improve their quality of life, electromyogram (EMG) prosthetic hands have been developed to compensate for some daily activities which a cosmetic glove cannot do. However, most EMG prosthetic hands pursue the ability of motions by multi-degree of freedom mechanisms that lead to the shortcomings, such as low intensity, high cost, overweight, and maintenance difficulties at the same time. The development of low-degree of freedom EMG prosthetic hand with high performance is therefore very practical and important. We have developed an EMG prosthetic hand with two motors to realize the rotary motions which account for 85% hand motions in daily life. In this study, we improve the mechanism of the hand with springs to connect the fingers to the palm and artificial nails equipped on top of the fingers. Experimental results show a significant improvement of grasp ability, especially for the tiny objects.