Abstarct: One of the oldest means of transmission is string. String twisted around itself or woven into strings, used for transporting force. When the electric motor is connected to one end of the string and the other end is connected to the intended load, applying rotation to the string through the engine is causedstring twists, decreasing the length of the string and consequently the load transfer, due to tensile force produced is. This transmission driving system called twisted string actuator systemhas many applications in robotic devices, due to low weight, cost and friction. Proposed flexible dynamic system model, includes a string which is connected to electric motor from one and to mass (robotic arm) from the other side. Through rotating the engine, string twists and its length changes caused concentrated mass movement. In order to find answers to the system, discrete method of transfer matrix is employed. Using discrete time transfer matrix method, system model has been studied in two modes. In the first model, the torsional spring and mass elements and in the second model, the elements of the mass, torsion and linear springs are intended and the results are compared with relevant research. By applying the PID system controller, steps response will be obtained. In the following by taking the system as a twisted beam, exact solutions for the dynamic characteristics of a twisted beam through transfer matrix method, in a special mode of modeling the spring-mass elements with evaluation of natural frequencies and mode shapes approach have been investigated.