Abstract: The coating and jacket of an optical fiber provide protection against brittle fracture. However, due to the stuff differentia, the existing of the coating and the jacket imposes influence on the sensitivity of the optical fiber to some extent. In order to accurately measure the strain applied on a structure, a theoretical model of distributed fiber optic sensor with coating and jacket is established from the view of microscopic mechanics. The influences affected by shear modulus and radius of the coating and jacket on the distributed strain measurement are analyzed based on this model, and the fiber strain sensing law was compared with different length of laid fiber. Some methods used to reduce the effect of coating and jacket on the strain measurement are presented in the end.