The dispersion of optical material influences the accuracy of lens gap measurement. In order to improve the accuracy of lens gap measurement, the effect of optical material dispersion is investigated by the theoretical analysis and numerical simulations. An expression of interference signal offset is obtained by numerical simulation for materials of LAK, ZF6, H- ZF2, BAK7 and K9，with the offset proportional to the first order dispersion coefficient of optical material. An experimental platform is set up to verify the simulation result, and it is shown that the relative error of interference signal offset between the experimental and simulation results is below 2% . Considering various error factors in the experiment, the relative error of 2% is reasonable and the obtained expression via simulations is correct.