This research collects 3896 errors made by 965 Chinese students in 5 junior middle schools in Xining and then a detailed analysis is made to discover the possible causes behind these errors. This research comes to the following conclusions:first, during the course of English learning in junior middle school, the students make errors mainly in six specific areas:spelling, tenses, pronouns, verb"be", articles, and numbers;second, the learners at an elementary level produce more interlingual errors than the learners at an advanced level. Third, error fossilization is serious.