In order to monitor the film thickness of polyethylene terephthalate (PET) composite substrate on 3-D glass, modeling analysis method of PET composite substrates equivalent to single-layer substrates was adopted. The thickness measurement of titanium dioxide gradient refractive index material thin films on complex substrates was realized by ellipsometry. The result show that, the thickness of titanium dioxide gradient refractive index films on PET composite substrates is 212.48nm by this method. The results of scanning electron microscopy is 211nm. The result is very accurate. The equivalent substrate method is validated by taking titanium dioxide as an example. The method is also applicable to other dielectric films. This method can measure and characterize the thickness of titanium dioxide thin films on PET composite substrates with high accuracy, and is of great significance for monitoring the coating process.