Compared with the Internet predominated by personal computers, mobile Internet has triggered faster, more wide-ranging and more extreme group polarization. This study takes Jiang Ge case as an example. Jiang Ge case refers to online discussions about a murder case in which Jiang Ge, a student studying in Japan, was killed by the ex-boyfriend of one close friend. Data of this study was collected from four mobile applications – toutiao.com, Tencent News, WeChat and Weibo, and covers all the media texts and netizen comments involving Jiang Ge case from November 9th to December 31st, 2017. This study finds that there existed obvious group polarization in the discussion of Jiang Ge case in the mobile Internet, and that negative emotions, such as “anger”, is ubiquitous in this discussion. Based on Jiang Ge case and some other typical group polarization events in the past few years, this study fnds that under the in?uence of source event stimulation and instigation of we-media, netizen crowds with new media usage habits in the mobile Internet environment can cause a hypnotic impact on each other, accelerating the formation of extreme tendencies, the spread of extreme emotions and the solidifcation of extreme attitudes.