Based on the basic principle of fiber optical Bragg grating (FBG) strain sensing and the basic theory of viscoelastic mechanics,a simplified time-dependant optical-mechanical transformation equation for FBG sensors is established. With the viscoelastic mechanic behavior of encapsulation matrix and optical fiber core being neglected and the elastic-viscoelastic mechanic coupling mechanism among environment-encapsulation matrix-adhesion layer-optical fiber core being taken into account,the equation hereby established contains all the characteristic geometric and mechanical parameters of a sensor,and can greatly simplify the process of optical-mechanical transformation analysis,thereby more reliable and less complicated for long-term performance analysis of FBG sensors when applied to health monitoring of large project.