In recent years, influencer marketing has gained increasing popularity, with many influencers embedding product information into their content (e.g., videos and articles). When fans encounter these messages, they may make unplanned purchases, resulting in impulse buying behavior, a long-standing issue in marketing research. This study aims to explore the factors that lead to such behavior. Using the Stimulus–Organism–Response (S-O-R) model as a framework, the study investigates how interactions between individuals and influencer content (Stimuli) trigger psychological changes in consumers, namely positive affect, flow state, and emotional attachment (Organism), which in turn lead to impulse buying behavior (Response). The study surveyed fans who had previously purchased products recommended by influencers, collecting 404 valid responses. The findings reveal that: (1) Consumers’ psychological changes (positive affect, flow state, and emotional attachment) significantly and positively influence impulse buying behavior. (2) Scarcity, discounted price, review quality, and observational learning also have significant positive effects on impulse buying. (3) Social presence and sense of belonging significantly enhance flow state. (4) Entertainment and informativeness significantly enhance emotional attachment.