This research sought to examine the impact of electronic portfolio -based writing instruction on the second language writing performance and writing self-efficacy of Chinese English as a foreign language (EFL) learners. A total of 64 participants were divided into two groups, an experimental group (n = 31) and a control group (n = 33). The experimental group was given electronic portfolio-based instruction via Edmodo platform, while the control group received only traditional in-class writing instruction without the use of Edmodo. Two sample IELTS tasks and Writing Self-efficacy scale were administered to collect the quantitative data. Further, semi-structured interviews were carried out with participants in the experimental group to gather qualitative data on their perceptions and experiences with electronic portfolio-based writing instruction. The results of paired samples t-tests and ANCOVAs indicated that electronic portfolio-based writing instruction significantly improved global writing performance and its underlying components, namely content, organization, language use, vocabulary, and mechanics, as well as the writing self-efficacy of Chinese EFL learners. Qualitative results revealed several themes, including increased motivation, improved feedback, enhanced collaboration, improved writing skills, and increased writing self-efficacy. Overall, the study demonstrates that electronic portfolio can be a valuable tool for EFL writing instruction and has a positive impact on writing performance and writing self-efficacy for Chinese EFL learners.