This study collects 3896 errors made by junior middle students in Xining. Then they are classified into interlingual er?rors and intralingual errors and a macro-analysis is done to discover the possible causes behind these errors. This research comes to the following conclusions:First, a large number of the errors made by the students are intralingual errors rather than interlingual ones. Secondly, the students at elementary levels produce more interlingual errors than those at intermediate or advanced levels. Conversely, the students at intermediate or advanced levels produce more intralingual errors. Thirdly, errors can have more than one source.