Short-term social integration at the initial stage is particularly important for first-year college students and often plays a key factor in determining newcomers' subsequent socialization outcomes. Recent years have witnessed the growing popularization and impacts of online social integration (OnSI) among college students. Drawing on dominant theoretical perspectives on the impacts of ICTs (information communication technologies), this study aims to reconcile previous mixed effects of OnSI on new college students' social adjustment at the initial stage. One hundred fifty-seven college newcomers were recruited and asked to report their daily social integration and experiences over ten consecutive days soon after they entered university. Results from multi-level analyses revealed that the relationship between OnSI and social adjustment was not significant; however, daily offline social integration (OffSI, i.e. participating in offline social activities) was positively related to social adjustment, which in turn, increased person-group fit and life satisfaction. Besides, we also found that OffSI moderated the relationship between OnSI and social adjustment such that, the relationship was positive when OffSI was at a low level but negative when it was high. This research delves into the effects of OnSI and OffSI on the initial short-term socialization experiences of first-year college students and further explores their implications for first-year students' social adjustment.