Abstract: The optical properties of optical thin-film are related to each-layer thickness closely,in order to acquire the optical thin-film product satisfying the requirement,the thickness must be monitored during the course of deposition. The monitoring accuracy of the optical thin-film determines the accuracy of deposited film thickness. Aiming at the defects that the electro-optical extremum method is difficult to monitor the extreme points and the non-normalized coating system accurately, we presented a new algorithm for optical thin-film thickness monitoring.Though the mathematical calculation, the relationship between film thickness and transmittance turns to be linear, and this new algorithm can eliminate the bad effects of optical source fluctuation and transmission noise. The accuracy of this new algorithm is less than 2%.