Paraphrasing, or restating information using different words, is critical to successful writing. However, EFL learners have difficulty in making paraphrases to meet their writing demands, and there has been little research on developing automatic reference tools to assist these learners' paraphrasing skills for better writing quality. In this study, we developed PREFER, an online corpus-based paraphrasing assistance system. Allowing multi-word input and returning promptly with a list of paraphrases in English and Chinese, along with usage patterns and example sentences, "PREFER" provides substantial support for EFL learners to vary their expressions during writing. An assessment study of the effectiveness of "PREFER" was conducted with 55 Chinese-speaking EFL college freshmen in an Asian country. The results indicated that "PREFER" offered the most benefits to students' writing performance (with an after-use improvement of 38.2%), compared with an online dictionary and an online thesaurus (-31.6% and -6.2%, respectively). Further investigation revealed that the less proficient, more motivated, and more conservative students showed more significant progress in the paraphrasing task with the help of "PREFER." In the meantime, nearly 90% of the students expressed satisfaction with the paraphrases generated by "PREFER," and its functions, and another 75% of them acknowledged that "PREFER" benefits their writing task.