A thickness measurement device based on low-coherent fiber-optic interferometry is proposed to solve coupling loss in thickness measurement system by using zoom lenses. According to optical fiber coupling condition and beam energy distribution，the light beam coupling efficiency influenced by the transverse size of the light beam and the curvature radius of surface is analyzed. Enhancing effect of coupling efficiency is theoretically analyzed when zoom lenses are applied to thickness measurement device. The experimental results show that the signal intensity is obviously improved via the introduction of zoom lenses and the relative measurement error of device is less than 0.05%.