The mechanic model of surface FBG sensor is set up.The relationship between the sensing strain and actual strain is deduced.The effect of substrate thickness and bond length on strain transferring is simulated.The sensitivity of surface FBG sensor is respectively 54.4pm/0.1mm，54.8pm/0.1mm，52.8pm/0.1mm when bond length is 40mm with substrate and without substrate and the bond length is 20mm with substrate.The experiment result is proved that the theoretical deduction and simulation are correct and provide guidance for sensor package.