Focusing on the problem that PrePost algorithm needs to build complex Pre-order and Post-order Code tree (PPC-tree) and Node list (N-list), an improved frequent itemset mining algorithm based on the Interval list (I-list) was proposed. Firstly, data storage structure with more compression compared to Frequent Pattern tree (FP-tree), called Interval FP-tree (IFP-tree), was adopted, which mined frequent itemsets without iteratively establishing conditional tree. Secondly, the more concise method called I-list was used to replace the complex N-list in PrePost so as to improve mining speed. Finally, in the case of single branch path, some frequent itemsets were directly obtained by the method of combination. The experimental results prove the correctness of the proposed algorithm by getting the same results for the same dataset under same minimum supports, the proposed algorithm is superior to PrePost algorithm by about 10 percent in terms of time and space which has a good application in sparse database or intensive database.