Motion strategy analysis at the pre-bending stage is a fundamental component of underactuated finger grasp research. This study presents the pre-bending motion strategy and corresponding analysis of cable driving forces a 3-DOF underactuated finger comprising cable truss units. This robotic finger uses a tendon-pulley transmission and parallel four-linkage mechanism to realize the grasp capability. The structure and four motion strategies at the pre-bending stage are illustrated. The equivalent joint-driven and quasi-static motion models are established in the case where one or two cable driving forces drive the finger. In accordance with the virtual work principle, the tendon-pulley transmission is transformed into an equivalent joint-driven system. On the basis of the constraints of maximum motion space of the finger, the joint spring stiffness distributions are discussed and the finger quasi-static motion space is analyzed under the condition of single motor driving force. The unique coupled motion process and corresponding cable driving force of the finger driven by a single motor are assessed. Furthermore, three other typical quasi-static motion strategies and their corresponding cable driving forces are discussed. Valid simulation experiments are conducted to verify the accuracy of the quasi-static motion strategy. The analysis of this study can provide guidance and a theoretical reference for the design of cable-driven underactuated hands and control of the couple-driven underactuated mechanism.