The invention discloses a method and system for recommending query based on user log. The method comprises acquiring an effective query log set according to the data set in the user log; selecting a typical query string as the training set, extracting 6 characteristic indexes of each query string in the effective query log set, such as support degree, popularity, recommendation degree, co-occurrence degree, similarity, and association degree, and constructing a composite prediction model based on the training set; and extracting the 6 characteristic indexes of candidate query strings inputted by a user, inputting the extracted characteristic indexes into the composite prediction model as variables, calculating the relevancy between each candidate query string and a given query string, and outputting n query strings with higher rank. The system comprises a data preparation module, a prediction model construction module, and a processing output model for realizing the above method. By fully utilizing the user log of a search engine, the method and system can recommend query strings with higher quality for the user.