Massive population agglomeration trends pose fiscal challenges for local governments. With the help of Mobile Internet data, this paper demonstrates how demographic changes shift the implicit debts of local governments. In China, municipal corporate bonds (MCB) constitute the most implicit debt, functioning as an out-of-budget financing tool guaranteed by local government. This paper finds that net population flow is a crucial driving force behind local government’s MCB debts. However, the relationship is highly asymmetric, only observable in the growing cities. We ascertain that the city’s implicit debts could alleviate the disparities in public amenity construction needs resulting from demographic shifts.