The session in search engine log denotes a sequential of queries from a user when he is searching for certain information during a period of time.Session analysis is a fundamental work for user behavior analysis.In contrast to current query log analysis focusing on the query modification and the relationship among user,query and clickthrough,this paper presents user behavior analysis based on session,which includes the times of reformulation and the relationship among user,query and click-through.Experimental results show that the way users reformulate queries relates closely to the times they reformulate,while the number of click-through is steady;the last query often gets longer than the first,what is more,the main variation lies in some changed characters;the more often the users use the search engine,the greater the likelihood that they reformulate queries.