Experiments to study the optical response characteristics of plastic optical fibers under shear and torsion conditions are conducted respectively which are correspond to the conditions when sliding type and tearing type cracks occurred in structures. Results indicate that during the process when the shear displacement increase from 0 to 0.5 mm, the optical loss and Fresnel reflection are linear related to the shear displacement, and the sensitivity of which are 15.6 dB/mm and 5.9 dB/mm respectively, indicating that plastic optical fibers have better optical sensibility to cracks under shear loading conditions, while the sensitivity is low under torsion loading, the optical loss of which is about 1 dB. It can be concluded that plastic optical fibers have the same optical response characteristics when sliding mode or tearing mode cracks occurred in structures. Both Fresnel reflection and optical loss can be used as crack monitoring index among which optical loss is considered to be more sensitive.