Student mobility is one of the most important indicators to reflect institutional internationalization in a sustainable higher education system. Student mobility issues have been addressed in previous studies, and the phenomenon was discussed in association with related factors persistently. Since higher education sustainable development has received much scholarly attention, monitoring student mobility flows to adjust international strategies is necessary. This study explored practical approaches to detect student mobility flows in the process of higher education expansion. Targeting Taiwan’s higher education system as an example, we addressed the topic of system expansion and the core issues of student mobility. Target series data were collected from 1950 to 2021, including the economic growth ratio, GDP per capita, higher education enrollment, gross enrollment ratio (GER), and the number of inbound and outbound students. The data were transformed with index formats, for example, the economic growth ratio, enrollment increasing ratio (IR), and net flow ratio. The cross-correlation function (CCF) and autoregressive integrated moving average (ARIMA) were used to determine the correlations of the series data and their future trends. The findings suggested that the system expansion, with GER and IR, might reflect fluctuated student mobility in economic growth. This study confirmed that the time series approaches work well in detecting the phenomena of higher education expansion and their effects on student mobility flow in the future.