This paper presents a novel under-actuated (UA) finger with first coupled and secondly self-adaptive (COSA) grasping mode. COSA fingers can adaptively grasp objects with different sizes and shapes while its motions during grasping are anthropopathic. Until now there are two COSA mechanisms available and they are both direct parallel combinations of coupled mechanism and self-adaptive mechanism. These kind of direct combinations lead to complex mechanical structure and high power consumption. This paper proposes a novel single-route transmission mechanism for COSA grasping mode, S-coupled and directly self-adaptive (CDSA) mechanism for short. Compared with available COSA mechanisms, the S-CDSA mechanism has simpler structure and higher grasping force. Design of 2-joint S-CDSA finger is introduced in this paper. Force analysis for 2-joint S-CDSA finger is given. Furthermore, a 2-joint S-CDSA finger is manufactured. The force analysis and experimental results show that the novel S-CDSA mechanism is effective.