In order to fit in with the Chinese language characteristics in the QA systems, this paper thoroughly analyzes the information retrieval model. After analyzing and comparing the traditional main IR models, we get a more efficiency IR method, which is SLMIR (an information retrieval method based on statistical language modeling). In addition, we study the best order number N in Ngram and its main data smoothing techniques, compare them by test results, and discusse the relevant factors which affect the data smoothing method,such as the scale of training. Finally, the best smoothing techniques in different conditions are given.