A new method is proposed to improve the monitoring accuracy of the thin film thickness in the photoelectricity extremum method. By improving the accuracy of the interpretation points, the random errors occurred when film coating is stopped in the optical extremum method are avoided. The nonlinear relationship between the monitoring signals and the optical thickness is transformed into a linear one and the optimum starting time is deduced by the treatment with algorithms, which avoids the influence of the nonlinear errors on the discrimination of extreme value points in the process of the monitoring thin film deposition.