To estimate the damage state of reinforced concrete (RC) members, a damage model was established based on stiffness degradation and fiber beam-column elements in Matlab. Firstly, the numerical analysis model of a member or a structure based on fiber beam-column elements was established in OpenSees, and the strains and stresses of fibers outputted by OpenSees were then read into the damage model established by Matlab, which estimated the damage values at fiber, section, and member levels. In the established model, fiber damages of concrete, reinforcing steel, and prestressing tendon were defined by the initial reloading modulus degradation, low-cycle fatigue law, and plastic strain, respectively. The section and member damage states were then evaluated by the degradation of the sectional bending stiffness and rod-end bending stiffness, respectively. The established model was verified by comparison with a structural test results of prestressed concrete frame subjected to cyclic loads. The results indicate that the damage model can determine its damage states accurately, and a close relationship between damage indices at various levels by using the direct stiffness method and static condensation method. Furthermore, the established damage model can be embedded in OpenSees to determine the damage states of RC members and structures directly in the future.