In thin-film thickness wideband monitoring system based on merit function, because the measured spectrum curve deviates from the designed one, the merit function becomes divergence, which results in the failure of monitoring. Utilizing the thin-film transmittance spectrum curve measured, for the thin-film layer which has been deposited, its actual optical constants are fitted in real-time by simulated annealing algorithm to correct the merit function. Then the absorption effect of layers having been deposited on the layers to be deposited is compensated, and the layer number and thickness of the layers to be deposited are redesigned. The new merit function is achieved, by correction layer by layer. Experimental results show that the monitoring error of thin-film thickness may be less than 10-2, and the precision satisfies the practical requirement.