The expansion of C-4 plants is one of the most prominent vegetation changes in the global ecosystem during the Cenozoic Era. Although C-4 plant expansions in the latest Miocene have been widely reported, factors driving the expansions are still in debate, and the details of vegetation changes during the Quaternary have not been well studied. Here we present high-resolution carbon isotope time series of both organic matter and bulk carbonates, covering the past 2.58 Ma, derived from the loess-soil successions on the southern Chinese Loess Plateau. The organic matter delta C-13 values indicate stepwise C-4 plant expansions initiated at similar to 1.6 and at similar to 0.43 Ma, respectively. We conclude that such tectonic time scale C-4 plant expansions are controlled by enhanced seasonality of precipitation (relatively more precipitation in the warm growing season) as well as regional aridity, and this long-term fluctuation superimposes on the orbital scale variations of C-4 plants, while the latter appears phase-locked with cyclical changes of summer monsoon circulations.