Grassland species diversity is closely linked to climate patterns. Prior research has predominantly focused on the ecological systems of northern secondary grasslands, leaving the connection between species diversity and climate in southern tropical secondary grasslands unclear. This study examined the species composition and diversity indices of the primary tropical secondary grassland vegetation in Hainan Island by utilizing the fundamental species information from 43 plots measuring 30m×30m situated in six cities and counties. In conjunction with 19 climate variables characterizing temperature and precipitation obtained from the Worldclim database, the study investigated the correlation between species diversity and climate factors. The findings shed light on the climatic driving forces impacting the diversity of species in tropical secondary grasslands in Hainan Island. The results showed that: the species richness of major tropical secondary grasslands in Hainan Island was significantly positively correlated with the monthly mean diurnal range (BIO2), isothermality (BIO3), and temperature annual range (BIO7) (P < 0.050); Simpson index was significantly negatively correlated with annual mean temperature (BIO1), min temperature of the coldest month (BIO6), and mean temperature of the coldest quarter (BIO11) (P < 0.050); the Shannon-Wiener index was significantly negatively correlated with the min temperature of the coldest month (BIO6) and significantly positively correlated with the temperature annual range (BIO7) (P < 0.050); precipitation factors (BIO12, BIO13, BIO14, BIO15, BIO16, BIO17, BIO18, BIO19) showed no significant correlation with the species richness, Simpson index, and Shannon-Wiener index of major tropical secondary grasslands in Hainan Island (P > 0.050). The findings of this study have revealed that the tropical secondary grasslands in Hainan Island have acquired the ability to adapt to the prolonged hot weather conditions characteristic of the tropics. However, when there are sudden significant temperature fluctuations or conditions of extreme cold, the species diversity of the tropical grasslands will be substantially impacted. These results could serve as a reference for investigating how species diversity in tropical secondary grasslands respond to climatic changes.