StackExchange is one of the most popular Q&A rendezvous, including dozens of Q&A communities. In the paper, based on the Q&A relationships and the geographical information of users, this paper analyzes the knowledge spreading patterns in Q&A communities in StackExchange in USA and statistical characters of networks. It extracts spreading patterns of Q&A communities and obtains knowledge sharing methods of users. We find that, generally, a Q&A community may have several spreading sources. Meanwhile, it establishes a spreading network for each Q&A community and finds that most of these spreading networks share similar statistical characters, indicating that these Q&A communities have similar spreading patterns.