A new radio frequency(RF) fingerprint extraction method is proposed for Airmax devices with proprietary protocols. Firstly, the construction of software and hardware experimental environment and the Airmax technology are introduced. Then the extraction method of the frame preamble signals is introduced, which is divided into two rough positioning and precise position. And the extraction method of Airmax RF fingerprint is expounded from theoretical analysis and experimental verification. A total of 14 dimensional features are extracted, in which 2 features are related to the frequency and 12 features are related to the amplitude. Finally, based on the 14-dimensional features, the K-means algorithm and the decision tree model are used to train and classify the data of features, and the classification precision is calculated. The precision of both models reach 100%. For the classification problem of four devices, the precisions of the K-means algorithm and the decision tree model are 92.4% and 100%, respectively.