In 2017, the Minhang District of Shanghai introduced sex offender registration. This reform attracted positive reactions on social media. Local governments in Jiangsu, Guangdong and Chongqing quickly followed the precedent. In 2019, the central government announced that it will establish national sex offender registries by 2022, although it limited the scope of registration to paedophiles. This study explores how this bottom-up reform unfolded and what implications it has in theory and practice. Based on qualitative and quantitative analysis of 2261 posts on Weibo, the Chinese version of Twitter, this study reveals two major players in the current reform: the Chinese feminists who are pursuing a punitive agenda online (a phenomenon known as 'carceral feminism'), and the local governments that are eager to win the public's trust, although such trust is mainly reserved for the central authority in the Chinese political culture (a culture known as 'populist authoritarianism'). Drawing on the findings, this study pushes the discussion about Chinese penal policy beyond the dichotomy of 'penal professionalism' and 'penal populism'. It argues that while the influence of professionalism is evident in central-level policy making, local penal policies can be easily led by populist punitiveness. The latter deserves more academic attention. This study also takes a non-partisan approach to the feminist movements on Chinese social media. It reveals the class conflicts and regional inequality underlying the gender schism. The polarisation effect in online discussion is also highlighted, which alerts policy makers to the reliability of 'public opinion' online.