Abstract: 【Objective】To reveal the characteristics of diameter microvariation of Castanopsis hystrix trunk in rainy weather, and to provide some basic datas for further research on the response of micro-growth of the trunk diameter to the global climate change.【Method】The experimental plot was within the area of Youyiguan Forest Ecosystem Research Station in Guangxi.Stem diameter microvariation was monitored for three C. hystrix trees using electronic point dendrometers. The instruments were installed on the tree trunks at breast height. Stem diameter microvariation was recorded by a data logger during the period from May to October 2019, and environmental factors such as wind speed, air temperature, rainfall, air relative humidity, photosynthetically active radiation, soil moisture content and soil temperature were measured simultaneously. The characteristics of diameter microvariation of the C. hystrix trunk under different rainfall conditions were revealed, and the correlation analysis method was used to analyze the correlation between the trunk diameter microvariation and environmental factors under different rainfall levels.【Result】In 2019, precipitation was 847.90 mm in the rainy season in this area, mainly with rainfall of <10 mm, accounting for 46.88% of the total rainfall amount. The ≥ 50 mm rainfall contributed the most to the total rainfall, accounting for 59.19%. During the monitoring period, the average daily micro-growth of C. hystrix trunk diameter was 30.65 μm, and the total diameter growth was 4383.17 μm, which was greater than the average daily micro-contraction(-19.91 μm) and the total diameter contraction(-796.44 μm). Rainfall had a significant(P<0.05, the same below) or extremely significant(P<0.01, the same below) effect on diameter variation, and the influence of different rainfall levels on diameter microvariation showed that:the peak occurrence time of daily cycles appeared to lag with increasing rainfall levels, while the valley occurrence time appeared to advance with increasing rainfall levels, meanwhile, the daily growth amplitude of diameter microvariation showed a decrease with increasing rainfall levels, and the daily cycle of microvariation in stem diameter increased monotonously under higher rainfall. Under the condition of continuous rainfall, the diameter of stem continued to expand. When the moisture content of stem reached saturation, the maximum daily diameter appeared. After a few days of continuous rainfall, the diameter appeared negative growth. In rainy weather, there was a significant or extremely significant positive correlation between diameter microvariation and air humidity, and a significant or extremely significant negative correlation between diameter microvariation and air temperature, saturated vapor pressure, soil temperature, and the correlation between diameter microvariation and wind speed, photosynthetically active radiation, soil water content was different with different rainfall classes.【Conclusion】Rainfall has significant effect on diameter variation, and diurnal variation characteristics of diameter has different responses to rainfall classes and rainfall duration. Among the related environmental factors, air relative humidity, soil temperature and saturated water vapor pressure have the most significant contribution to the diameter variation, and they have a controlling effect on the diameter microvariation of C. hystrix trunk under different rainfall conditions.