This paper aims to explore the influences of L1 in the aspect of writing in L2, and it serves as a qualitative case study to disclose how Chinese as L1 influences writing in English (L2) in the context of IELTS writing exercises as part of mock exams. The influences of concern include: First, the study objects habitual or frequent misuse and mistakes of the L2 resulting from L1s existing language system; these errors can be grammatical, lexical, syntactic, structural, word choice, tense, etc.; and second, how the study objects previous experience of writing similar texts in L1 is extending its influence in L2 writing. The data were collected from students who used to prepare for IELTS academic tests to achieve the ends mentioned above. Writing tasks prepared for IELTS mock exam were collected for error collection and in-depth word-by-word analysis. The data for the study were collected after the IELTS exam was taken; these essays were part of the preparation for the IELTS exam. Therefore, the analysis can be used as a good indicator of how students perform and write formally in an academic context, not to mention the pressure experienced during the exam. The data analysis shows that there is a wide range of mistakes or misuses in writing in L2, all of which show a link to the interference or transfer of L1 to writing in L2 in aspects of grammar, semantics, and syntax. The interference results in ungrammatical or broken English. The analysis also reveals that the differences between L1 and L2 language systems may have resulted in some common errors and the so-called Chinglish. On the one hand, the linguistic habits and thinking habits of L1 possibly interfere with writing in L2. On the other hand, students unfamiliarity with the English language, including grammatical rules, semantic structures, and correct usage of words and phrases, also contributed to reduced efficiency or miscommunication of writing in L2. Based on the above-mentioned findings, suggestions are made for students on methods to improve L2 acquisition and application.