Abstract: 【Objective】This research was conducted to probe the stem sap flow characteristics of Cyclobalanopsis gracilis and the relationship between sap flow velocity and environmental factors in a Karst forest.【Methods】 The Thermal Dissipation Probe method and a Campbell Automatic Weather Station were used to conduct long-term continuous observations on the sap flow velocity of C. gracilis along with synchronized environmental factors such as air temperature and rainfall in the Maolan Karst original forest from March 2012 to February 2014. The changing characteristics of sap flow in different weather conditions(sunny days, cloudy days, and rainy days)and on different time scales(dates, months, and seasons), and its response to environmental factors were studied. 【Results】The findings revealed that in the whole observation period, the average daily sap flow velocity of C. gracilis was(5.78 ± 0.19)g/(m²·s), and the average daily sap flow velocity in relation to the different weather conditions was, in descending order, sunny days(10.35 g/(m²·s)), cloudy days(3.28 g/(m²·s)), and rainy days(3.16 g/(m²·s)). The average daily sap flow flux of the C. gracilis sample trees was(3.37±0.11)kg/d, with those on cloudy days and rainy days amounting to only 31.7% and 30.6%, respectively, of that on sunny days, which was 6.03 kg/d. The average daily sap flow velocity in relation to the different seasons was, in descending order,(8.28±0.38)g/(m²·s)in summer,(6.75±0.38)g/(m²·s)in autumn,(6.11±0.34)g/(m²·s)in spring, and(1.91±0.15)g/(m²·s)in winter. The air relative humidity(RH)was negatively correlated with the sap flow velocity. Solar radiation, air vapor pressure deficit(VPD), air temperature(Ta), and soil water content(SWC)were all positively correlated with the sap flow velocity.【Conclusion】The main environmental factors affecting the sap flow velocity were solar radiation, VPD, RH, and Ta. Mild SWC deficiency had no impact on the sap flow velocity. Each environmental factor’s impact on the sap flow velocity varied under different weather conditions and seasons.