Abstract: This study investigated the flowering phenology and breeding system of the epiphytic orchid Rhynchostylis gigantea (Lindl.) Ridl. on Hainan Island, China, based on two years of field surveys across seven native populations. Results showed that the flowering period of R. gigantea lasted from late January to mid-March, with a population flowering duration of 28–43 d, an average individual flowering time of 26–40 d, and a single flower lifespan of 7–12 d. The outcrossing index (OCI) value of 4 indicated a predominantly allogamous mating system with partial self-compatibility, reliant on biotic pollination. Both pollen viability and stigma receptivity were highest on the second and third days post-anthesis. Controlled pollination experiments demonstrated a significantly higher fruit set in outcrossing treatments (97.3%) compared to self-pollination (83.3%), whereas natural fruit set in the wild remained extremely low (4.2%). No fruit production was observed under bagging or emasculation experiments, confirming that R. gigantea exhibits a mixed mating system and lacks autonomous self-pollination or apomictic reproduction.