Relative clause is one of the most important language points in College English Examination. Teachers have been attaching great importance to the teaching of relative clause, but the outcomes are not satisfactory. Based on Error Analysis theory, this article aims to explore the reasons why senior high school students find it difficult to choose relative words and how to improve this situation. Participants in this study are 73 senior high school students from Tong Liao, Inner Mongolia. These students are given a test on using relative clause. Questionnaire on these 73 students and 30 teachers has also been carried on. After collecting and scoring the tests paper and questionnaires, using quantitative and qualitative method, the author finds the main causes of the students' choosing wrong relative words stem from: students pay much attention to language form and apply English rules mechanically, ignoring meaning and context of the language; students' grammar knowledge is not comprehensive and systematic and students tend to neglect or forget some grammar points; students lack necessary basic English grammar and vocabulary knowledge, resulting in the difficulty in understanding the meaning of the sentence; students can't adjust to relative clause because of language transfer. Therefore, teachers should strengthen the exercises of students' English basic knowledge, teach grammars systematically and comprehensively, encourage students to use effective learning strategies to learn English, find ways to increase the input and output of English language and train students think in English mode. As to the students, they should learn to reflect on themselves, strive to improve their English and correct their learning attitude, and learn to use learning strategies effectively under the help of teachers.