Student mobility is one of the most important indicators to reflect institutional internationalization in a sustainable higher education system. While student mobility issues have been addressed persistently, the phenomenon is rarely discussed in association with higher education expansion. 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 higher education enrollment, gross enrollment ratio (GER), and number of inbound and outbound students. The data were transformed with index formats, for example 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 effect of system expansion, with GER and IR, might influence the mobility ratios significantly in the process of higher education expansion. This study confirmed that the time series approaches work well in detecting the phenomena of higher education expansion and their effects on student mobility flows in the future.