Third-Party libraries are widely used in mobile applications such as Android apps. Much research on app analysis or access control needs to detect or classify third-party libraries first in order to provide accurate results. Most previous studies use a whitelist to identify third-party libraries and manually categorize them. However, it is impossible to build a complete whitelist of third-party libraries and classify them because:(1) there are too many of them; and (2) common techniques such as library obfuscation and library masquerading cannot be handled with a whitelist. In this paper, an automated approach is proposed to detect and classify frequently-used third-party libraries in Android apps. A multi-level clustering based method is presented to identify third-party libraries, and a machine learning based technique is applied to classify the libraries. Experiments on more than 130 000 apps show that 4 916 third-party libraries can be detected without prior knowledge. The classification result of 10-folds cross validation on sampled libraries is 84.28%. With the trained classifier, the proposed approach is able to classify more than 75% of the 4 916 libraries into six categories with an accuracy of 75%.