INTRODUCTION: The large number of key populations in China who would benefit from HIV pre-exposure prophylaxis (PrEP) in the context of limited health system capacity and public awareness will pose challenges for timely PrEP scale-up, suggesting an urgent need for innovative and accessible interventions. This study aims to develop and pilot test a theory-informed, tailored mobile phone intervention that was codeveloped by young gay men, HIV clinicians and public health researchers to increase engagement in PrEP education and initiation among Chinese gay, bisexual and other men who have sex with men (GBMSM), who bear a disproportionate burden of HIV infections and remain underserved in the healthcare system. METHODS AND ANALYSIS: This two-phase study includes a formative assessment using in-depth interviews (N=30) and a 12-week experimental pilot study using a two-arm randomised controlled trial design (N=70). The primary intervention is delivered through a WeChat-based mini-app (a program built into a Chinese multipurpose social media application) developed by young GBMSM from a 2019 crowdsourcing hackathon. Using mixed methods, we will further investigate the specific needs and concerns among GBMSM in terms of using PrEP as an HIV prevention strategy, how their concerns and PrEP use behaviours may change with exposure to the mini-app intervention during the study period and how we can further refine this intervention tool to better meet GBMSM's needs for broader implementation. ETHICS AND DISSEMINATION: This study and its protocols have been reviewed and approved by the Institutional Review Boards of the University of North Carolina at Chapel Hill, USA (19-3481), the Guangdong Provincial Dermatology Hospital, China (2020031) and the Guangzhou Eighth People's Hospital, China (202022155). Study staff will work with local GBMSM community-based organisations to disseminate the study results to participants and the community via social media, workshops and journal publications. TRIAL REGISTRATION NUMBER: The study was prospectively registered on clinicaltrials.gov (NCT04426656) on 11 June 2020.