Rhododendron griersonianum is a critically endangered plant species with extremely small populations (PSESP) in urgent need of rescue. Understanding the processes that lead to species endangerment is important for the implementation of effective conservation measures. Studying the breeding systems and elucidating the reproductive biological characteristics of an endangered plant species is a crucial step in identifying the mechanism underlying its endangerment and small populations. However, no previous study to date has investigated the breeding system of R. griersonianum. In this study, we conducted a preliminary study on the floral syndrome and breeding system of R. griersonianum at its type specimen collection locality, Houqiao Town, Tengchong City, Yunnan Province, China. We investigated the flower characteristics, reflectance spectra, nectar volume and nectar concentration, flower visiting insects, and conducted pollination experiments. The results showed that (1) R. griersonianum possesses a suite of floral syndromes that facilitate its adaptation to Papilio-mediated cross-pollination, including large flowers, long corolla tubes, small nectar volumes, red flowers and floral reflectance curve peaks ranging from 350 nm to 400 nm; (2) Papilio helenus and Papilio bianor are effective pollinators of R. griersonianum, Bombus flavescens and Sphecodes sp. are nectar or pollen thieves; (3) R. griersonianum has a pollinator-dependent mixed mating system that combines cross-fertilization with self-fertilization, and has low fruit set under natural conditions and pollination limitation due to pollinator scarcity. The findings of this study provide a theoretical basis and reference for the future conservation of R. griersonianum populations. They also serve as a valuable reference for the conservation of other rare and endangered species within the genus Rhododendron.