Spectral-domain optical coherence tomography (OCT) is used for the inner defect detection of the optical thin film with high accuracy. It is theoretical analyzed and experimentally validated. The two-dimensional (2-D) cross-sectional images of the optical thin film with single layer and multi-layer structure are obtained respectively. Based on the 2-D cross-sectional images, the defect in the film can be observed clearly, and the position of the defect in film can be located. It is demonstrated that spectral-domain OCT is an effective method for defect detection of the optical film.