Attributive clause reflecting the Hypotaxis characteristics of English language, is one of the most commonly used clauses in English. It has always been a big challenge in E-C translation. Based on the analysis of the machine translation of UNECE automobile regulations, common mistranslation types are summarized, and post-editing methods are discussed from three levels—the vocabulary level, the clause level and the sentence level, to provide some insights into improving the translation quality of machine translation.