Aimed to overcome the deficiency of abundant data to web mining, the paper proposed an association-analysis based algorithm. Firstly, we construct the relation Information System using original data sets. Secondly, make use of attribute reduction theory of Rough sets to produce the Core of Information System. Core is the most important and necessary information which cannot reduce in original Information System. So it can get a same effect as original data sets to data analysis, and can construct classification modeling using it. Thirdly, construct indiscernibility matrix using reduced Information System, and finally, get the classification of original data sets. The experiments shows that the proposed algorithm can get high efficiency and can avoid the abundant data in follow-up data processing.