China has entered a new era of high-quality advancement. Investigating the seasonal variation mechanisms of urban vitality in severe cold city represents both a practical requirement for enhancing livable resilience and a critical task for revitalizing Northeast China. Against this backdrop, this study examines the central urban area of Changchun as a representative case. By processing multi-source big data, we compare spatiotemporal distribution characteristics of urban vitality between winter and summer workdays. The geographically and temporally weighted regression model (GTWR) is used to reveal the spatiotemporal heterogeneity between the built environment and urban vitality in severe cold cities, and to analyze the impact of seasonal changes. The results show that: (1) Urban vitality in the central urban area of Changchun demonstrates strong spatial agglomeration, exhibiting monocentric concentric patterns during both seasons; (2) High-vitality durations last longer in summer than winter; (3) The association intensity between selected built environment features and vitality varies significantly, with floor area ratio (FAR) showing the most substantial enhancement effect; (4) The practical impacts of built environment on vitality are diminished during winter months; (5) High-FAR with low building density constitutes the optimal development paradigm for boosting vitality in Changchun's urban core.