Garuga floribunda var. gamblei, a common deciduous species in Xishuangbanna tropical karst forest, is susceptible to water stress. In order to determine the intra-annual xylem growth phenology of trees in tropical karst forest and how environmental factors would affect xylem formation, we monitored the dynamics of stem radius variation and xylem growth for G. floribunda var. gamblei by combining band dendrometer and microcoring during two consecutive years (2020-2021). Meanwhile, we analyzed the correlation of environmental factors with monthly radial growth and bi-weekly xylem growth, respectively. The results of microcoring method showed that enlarging zone began to appear in the end of March and wall thickening zone ceased in late September in 2020, whereas enlarging zone occurred in the middle of April and wall thickening zone stopped in the middle of October in 2021. However, the stem radius variation monitored by band dendrometer method showed that the radial growth of G. floribunda var. gamblei started later and stopped earlier than those of microcoring method in both 2020 and 2021. Annual radial growth of G. floribunda var. gamblei was higher in 2020 ((3.30±2.91) mm) than 2021 ((2.36±0.81) mm) in spite of longer growth duration in 2021. The durations of xylem growth were similar in two years, and the annual xylem growth were also similar in 2020 ((2.87±1.46) mm) and 2021 ((2.98±1.02) mm). Monthly radial growth, the thickness of enlarging zone and the thickness of wall thickening zone were positively correlated to precipitation in 2020 and 2021, indicating that water condition plays an important role in xylem growth of G. floribunda var. gamblei; the thickness of enlarging zone was also positively correlated with daily mean air temperature and daily minimum air temperature during two years, whereas the correlation coefficient between the thickness of enlarging zone and daily minimum air temperature was higher in 2021 than 2020; the thickness of wall thickening zone was positively correlated to relatively humidity, soil water content at a depth of 20 cm, daily mean air temperature and daily minimum air temperature in both years, but all these correlation coefficients were higher in 2021 than 2020. Our results will be helpful to understand how the intra-annual xylem growth of tropical karst forest trees response to environmental factors and provide theoretical knowledge to predict the xylem growth trend in tropical karst forest under global climate change in the future by revealing the patterns of intra-annual xylem growth of G. floribunda var. gamblei in two years.