The pomp of online shopping carnivals (OSC) and herein the frenzy of impulse buying (IB) have been extensively reported in media but received insufficient attention in academics. This study researched both the IB formation and enactment stages in “Double 11” OSC by conducting a large-scale online survey. Regarding the formation stage, consumers’ social community and personal involvement were identified as significant factors in promoting the urge to buy impulsively (UBI) in the OSC. Specifically, consumers’ OSC involvement appeared as their carnival experiences including participation, interaction, and pleasure; The social community consisted of their connections with e-commerce platforms, e-merchants, the media, net friends, close friends, and relatives during the OSC. Regarding the IB enactment stage, the cognitive evaluation revealed either a positive or negative effect on moderating the UBI-IB relationship via the direct path or the outcome expectancy-mediated path, respectively. Meanwhile, outcome expectancy exerted a dominant mediating effect on the UBI-IB relationship as the cognitive evaluation became sufficiently negative. These results yield valuable insights into 1) the distinguished preconditions of the OSC context facilitating consumers’ impulse formation, 2) consumers’ impulsive and reflective psychological processes behind the IB decision-making. This study also contributes substantially to IB theories and practical knowledge of OSC.