Abstract: Pre-editing is an important way to improve the quality of machine translation through machine-human interaction. This paper, taking the Chinese translation of English user manual of iPhone5 as an experimental text, aims to investigate the influence of pre-editing in accordance with controlled language on machine translation (Google). It is found that: 1) pre-editing based on controlled language has a positive influence on the enhancement of both the quality of machine translation regarding adequacy (information content) and fluency of the target text; 2) pre-editing exerts a bigger influence on the improvement of fluency than on adequacy; 3) in order to produce a translated text of high quality, pre-editing itself is far from enough, and thus post-editing is needed to further improve machine translation output. Based on the experiment and example analysis, some pre-editing methods are also put forward in order to give some guidance to pre-editors, including those at lexical, phrasal and sentential levels.