Objective To study the effects of the lapel shaper on the film thickness when the packaging film went through the lapel shaper during the stretching process in the FFS packaging machines. Methods This paper researched the thickness change of the packaging film when it went through the lapel shaper from the perspective of geometric nonlinearity, and compared it with the initial thickness. Results By analyzing the formulas of the relative displacement, relative velocity and acceleration, it was found that with the increase in the relative displacement, relative velocity and acceleration, the film thickness decreased, and the amount of deformation increased. Conclusion Formulas of deformation gradient tensor, displacement gradient tensor, velocity and acceleration were given, and a formula on the thickness variation of packaging film was introduced. This research provided theoretical foundations for the shoulder surface offset problem of the lapel shaper in the actual production process.