Functional specification documents are very important for the developers who want to understand and reuse unfamiliar software libraries. Due to high cost of human effort and time, lots of software do not provide the official functional documentation. However, some software communication records produced in software developing processes contain valuable information regarding software functions and usages. In this paper, an approach is proposed to automatically mining and organizing functional features for open source software based on StackOverflow data. By describing functional features in the form of verb phrases, this approach generates hierarchical list of software functional features as the supplement of software documentation. In the experimental evaluation on some real-world subjects, the automatically generated documents have covered 97.6% of the frequent-used functional features in the official documents. At the same time, the proposed approach can be adapted to different types of software communication records, and applied to software in different domains.