The strain measurement by the fiber Bragg grating (FBG) is achieved by the Bragg wavelength change when the FBG receives axial strain. But,because of the transverse effect,the FBG' s position of installation will affect the result of the measurement,and the error of the installation will bring the error of the measurement. This paper receives the formula of the error which comes from the error of the installation through theoretical analysis. When the angle has existed in the axial of the optic fibre and installation positions needed,the greater the angle is,the greater the error is. The accuracy of installing position of optic fibre is very important,it concerns directly the accuracy of the measurement's result.