Mechanical properties of red blood cells (RBCs) play an important role in regulating cellular functions. Many recent researches suggest that the cell properties or deformability may be used as a diagnostic indicator for the onset and progression of some human diseases. Although optical stretcher (OS) has emerged as an effective tool to investigate the cell mechanics of RBCs, little is known about the deformation behavior of RBCs in an OS. To address this problem, the mechanical model proposed in our previous work is extended in this paper to describe the mechanical responses of RBCs in the OS. With this model, the mechanical responses, such as the tension distribution, the effect of cell radius, and the deformed cell shapes, can be predicted. It is shown that the results obtained from our mechanical model are in good agreement with the experimental data, which demonstrates the validity of the developed model. Based on the derived model, the mechanical properties of RBCs can be further obtained. In conclusion, this study indicates that the developed mechanical model can be used to predict the deformation responses of RBCs during optical stretching and has potential biomedical applications such as characterizing cell properties and distinguishing abnormal cells from normal ones.