The present invention discloses a film thickness monitoring method for optical thin films. The method comprises the following steps that: 1) the number of extreme points of the T or R of a film layerduring the increase of the film thickness of the film layer is monitored, so that a film layer of which the thickness is an integer with lambda/4 as basic units; 2) when n1*d1 changes, and dR/d(n1*d1)changes periodically, so that dR/dt can be adopted to replace dR/d(n1*d1) in differential processing on time t; 3) a proper number of data points near preset extreme points are selected to be subjected to straight line fitting, so that an optimal straight line equation can be obtained; the initial value of a is determined through using an end point method, and a* is searched through using a one-dimensional optimization method, an initial value a <(0)> is given by using an successive approximation method, and step length h is selected and is gradually adjusted until the step length satisfies arelation expression that |u(a<(k)>)-u(a<(k-1)>)|