To discuss the strain error caused by the relative slip between the flexible Fiber Bragg Grating sensor and the substrate material， ANSYS finite element simulation software was used to analyze the relationship between the relative error of axial strain at each point of the core and the forward pressure. The parameters affecting strain transfer of the sensor， including core material， coating material， substrate material and size were analyzed， and the effect of relative slip on the relative error of FBG axial strain were explored. The results show that the relative error of axial strain at each point decreases with the increase of forward pressure， when the pressure is in the range of ［0.1 N， 10 N］， which presents the trend of small in core and big in two ends. In practical application， it should be combine the positive pressure to select the core material with less elastic modulus and the substrate material with higher friction coefficient， the relative error of axial strain could be less than 10%； select coating elastic modulus 2.4×10¹⁰ Pa， 0.062 5 mm thickness， the relative error of axial strain could be reduced to 8.57%； when half length of the FGB is more than 40 mm， the axial strain relative error of the core could be less than 20%， at this point， it could be considered that the coating and silica gel are completely bonded.