With the vigorous development of the Internet, the continuous application of social media platforms has accelerated people's channels to obtain information. WeChat Public Platform (WPP), as a mobile learning platform, is derived from the current popular social platform "WeChat", which can transfer text, pictures, video, audio and other multimodal information. Based on various functions of WPP, combining it with language teaching is one technique to realize blended learning so that learners can experience a novel learning mode. Some issues have surfaced in the present Business English Writing (BEW) class for Chinese EFL undergraduates, including a lack of teaching time, students' psychological anxiety about BEW, and a rigid evaluation process. This study mainly applies WPP in Chinese EFL undergraduates' BEW class and investigates whether WPP can help improve students' BEW performance and enhance students' interest in learning BEW. Quantitative and qualitative research methods were adopted, and 80 Chinese undergraduates were involved in this quasi-experiment. By analysing the experimental results and data of the students' pre and post-test and the interview results, this study found that the English performance of the experimental class students who implemented the WPP-based BEW teaching mode has been remarkably improved, and their enthusiasm for BEW was gradually increased. Implications point to integration of WeChat in teacher instruction, teacher practice, ESL curriculum and assessment.