The recent studies of newly discovered Pongo fossils with precise absolute age brackets have deepened our understanding of their taxonomy, teeth size evolution and its relationship with environmental changes in southern China during the Pleistocene period. In the present study, we summarize the representative Pleistocene orangutan fossil-bearing sites in southern China and discuss the latest research progress on orangutan fossils. The orangutan fossil found so far and the chronological results indicate that orangutans first appeared in southern China in the early Early Pleistocene and lasted until the Late Pleistocene. From the results of the current studies, at least two species of orangutans may have existed in southern China during the Pleistocene period. One of them is Pongo weidenreichi that is characterized by its overall larger dental size, a high frequency of lingual cingulum remnants and a low frequency of moderate to heavy wrinkling on its molars. P. weidenreichi survived in southern China from the Early Pleistocene to the Late Pleistocene. The other orangutan species, Pongo devosi, may have first appeared in southern China during the late Middle Pleistocene. Compared with P. weidenreichi, P. devosi is distinguished by its relatively smaller overall dental size, relatively lower frequency of lingual cingulum remnants on its molars, the well-developed lingual pillar and lingual cingulum on its incisors and relatively higher frequency of moderate to heavy wrinkling on its molars. More fossil evidences are needed to confirm the relationship between P. weidenreichi and P. devosi in future study. The teeth size evolution of Pleistocene orangutans shows obvious stages. Metric data of orangutan teeth fossils (>1000) from mainland Southeast Asia sites reveal that the reduction in the size of orangutan teeth fossils occurred mainly from the Early Pleistocene to the Middle Pleistocene. And orangutan teeth fossils remained relatively stable in their sizes from the Middle to Late Pleistocene. The stable carbon isotope data of Early to Late Pleistocene mammalian fossil teeth from mainland Southeast Asia can be used to reconstruct changes in the paleoenvironment and could provide some clues to interpret dental size variation of Pongo assemblages in a broader temporal and environmental context. The carbon isotope data show that dental size reduction in orangutan fossils is closely linked to environmental changes. The dental size changes in orangutans appear to coincide with the expansion of savannah biomes and the contraction of forest habitats from the Middle Pleistocene onward.