The thermal diffusion sap flow probes were used to continuously measure the sap flow rate of sample trees of Pittosporum brevicalyx growing as subdominant trees species in the Maolan Karst forests from February 2012 to March 2014. Simultaneously, the environmental factors were measured including air temperature, solar radiation, precipitation and soil moisture, etc. The aims of this study are to understand the daily and seasonal variation of the sap flow rate and single tree transpiration of sample trees of Pittosporum brevicalyx, the characters under different weathers, and the relations between sap flow rate and environmental factors. The results indicated that the daily mean sap flow rate of Pittosporum brevicalyx was (3.14±0.10) g H2O/(m2 s), with the rate of (5.20±0.17) g H2O/(m2 s) in daytime and (0.55±0.02) g H2O/(m2 s) in night time, a difference of as high as 9.45 times The mean sap flow rate (g H2O/(m2 s)) under different weather conditions followed the order of sunny days (5.71±0.15)＞cloudy days (1.95±0.10)＞raining days (1.65±0.10). The mean daily transpiration of single trees was (2.10±0.07) kg within the variation range of 0.09 to 8.05 kg, within which (1.94±0.06) kg, transpired in daytime and (0.16±0.01) kg in night time respectively, accounting for 92.38% and 7.62% of the whole day transpiration. The mean daily transpiration (kg/d) of single trees in different seasons followed the order of summer (3.32±0.14)＞autumn (2.18±0.12)＞spring (2.14±0.12)＞winter (0.76±0.05). A correlation analysis showed that the transpiration rate of either the whole day or the daytime and night time under all weather days was significantly and positively correlated (P＜0.01) with the air temperature, VPD and wind speed, and significantly and negatively correlated with the air humidity. Furthermore, the sap flow velocity was significantly and positively correlated with the solar radiation at the whole day and daytime, the precipitation in rain days, the volumetric soil water content at the night time of sunny days and cloudy days and at any time of rain days. The stepwise regression equations were established between mean daily sap flow rate of single trees and the environmental factors according to time scales and weather types. The results of step wise regression analysis showed that the solar radiation and volumetric soil water content in 20 cm layer soil were the major environmental factors influencing sap flow in both daytime and nighttime. The air temperature had a stronger influence on sap flow than other environment factors in both sunny days and cloudy days; while the solar radiation was the major factors influencing sap flow in rain days. The sap flow was main controlled by solar radiation in spring, autumn and winter; while by the VPD in summer. The solar radiation, air temperature and VPD were the main factors influencing the sap flow rate of trees of Pittosporum brevicalyx, and the soil water content was the main driver of the sap flow in night time.