In order to know the different capacity of water cycle in the forest ecosystem,we monitored the trunk sap flow in different diameter at breast height (DBH) of Pinus sibirica which in sunny side and shady side using TDP (thermal dissipation method) constantly in the Kanas National Nature Reserve,northwest of Xinjiang.The aims were to explain the relationship between water cycle of tree and environment,and provide basis on forest ecosystem water cycle responding to the climate change in the regional scale.The results showed that:(1)the diurnal variation of sap flow in sunny day,rainy day,and cloudy day were all multi-peak type.There were significant changes on frequency and range among these three weather conditions.The sequence of daily maximum sap flow value was sunny day>cloudy day>rainy day.(2)There was the hysteresis effect between sap flow and photosynthetically active radiation,hysteresis time of the maximum sap flow was about 30-207 min,respectively.(3)From June to September,the sap flow of P.sibirica in the sunny side were greater than those in the shady side,and the sequence of monthly mean sap flow was July>August>September>June.(4)The maximum transpiring water-consumption of P.sibirica was in July,and the value of it was 61.8% of the total value of whole growing season.Total transpiring water-consumption of P.sibirica with big diameter was 6 716.79 g in sunny side,while 4 649.08 g in the shady side.It was the 2.00 times and 2.45 times small diameter of P.sibirica,respectively.(5)Air temperature,air relative humidity,and photosynthetically active radiation were significant factors that affected the sap flow.Otherwise,the soil temperature in the layer of 0-5 cm and 20-30 cm were highly impacted on the sap flow too.This research showed that both sap flow and transpiring water-consumption of P.sibirica with big diameter were higher than that of P.sibirica with small diameter in the process of growth;these changes mainly happened in the sunny side of the trunk,especially in July.