This study aims at investigating the effects of online collaborative prewriting via DingTalk, a social software based on computer-mediated communication (communication across two or more networked computers), on EFL (English as a Foreign Language) learners’ writing anxiety and writing performance. A total of 60 junior high school students were involved in the experiment. During the treatment, one class was required to adopt online collaborative prewriting in DingTalk, while the other class was asked to brainstorm alone. Written texts in pre-test and post-test were collected to analyze in detail. Meanwhile, second language writing anxiety inventory was administered in pre-test and post-test to examine the change of participants’ writing anxiety. A post-study survey and an interview were conducted to investigate participants’ attitudes and perceptions towards online collaborative prewriting. The results revealed that (1) online collaborative prewriting facilitates junior high school students’ overall writing performance, (2) online collaborative prewriting helps to reduce overall writing anxiety, (3) students generally hold positive attitudes on online prewriting activity.