Abstract:The style and the sentence structure of English patent are much the same as those of English for Science and Technology. Passive voice, a typical phenomenon in patent, which is also widely used in EST, is the main focus in the paper. As a special syntactic form, passive voice is believed to be constrained by some non-syntactic elements, including semantic constraints and textual constraints, which will be analyzed closely in the present research. 

　　　The paper will use the passive voice in English patent as examples to illustrate passive voice in ESP English. With regard to the aspect of semantic constraints, no need to show the agent, and to stress the result, are two main reasons why passive voice is used. concerning the aspect of textual constraints, passive voice can help keep sentences statements objective, build clear structure, and create coherence in a text. With all these characteristics, passive voice helps make an article concise, impersonal, and formative, which is accord with the function of English patent, and it is the reason why it is largely used by patent writers. 

Key Words: Invention English patent; passive voice; objectivity

摘要:英文专利（写作句式严谨，表达方式虽未严格要求定义，但在百年专利写作的开拓与摸索中，已经积累形成具有自己特色、句式结构和表达方法。在写作手法和句型、句式的应用上与一般科技文类似，被动语态在其中得到了充分的应用。本文将提取被动语态这一典型的科技文语言现象作为研究对象，从语义和语篇这两个角度对其加以分析。

　　　本论文就英语科技专利中的表达需要作具体分析并解释科技发明专利中的被动语态案例。从语义角度,被动语态应用的主要原因有:无需现实主语、强调试验结果；从语篇角度，被动语态可以保持叙述的客观性，构建简明的句子结构，并有助上下文的连贯。专利文件写作结构严谨，句式表达简单清晰。而被动语态的运用能使文章达到叙述客观、表达正式等效果，符合专利英语特点，所以在科技英语中被广泛接纳和使用。

关键词：英文发明专利；被动语态；语篇限制；语义限制