Abstract: The thermal dissipation probe was used to continuously monitor the sap flow of Cryptomeria japonica in different DBH in the Lushan Nature Reserve in July 2016, and monitored the meteorological factors (rainfall, temperature, humidity, solar radiation, etc.) in the meantime.The sap flow change rules and transpiring water consumption characteristics of Cryptomeria japonica with different DBH and its response to the changes of main meteorological factors were revealed. The results show that the flow pattern shows obvious diurnal variation, and the diurnal variation of the flow is multi-peak curve, the average number in sunny peaks is about 3.5 times, the foggy day is 3 times. In the typical sunny day, the average daily start time was at 6: 45, the total duration was 15 h, the first peak appeared at about 11: 45, the peak duration was 4.25 h. In foggy day, the average daily start time of the sap flow was at 7: 30, duration was 11.5 h, the time of the first peak appear was about 10: 45, the peak duration was 4.5 h, and the peak time of the sap flow, the falling time, the finish time and the highest peak time were longer than sunny days. The diurnal variation of the sap flow in the DBH was different. On the whole, with the increase of the DBH, the number of times of sap flow fluctuation was increased, the first peak and end time of sap flow in different trees are different, the highest peak of the sap flow increased with the increase of the DBH. In sunny day, the mean of the sap flow and the DBH had a good linear positive correlation, while in foggy day, the linear correlation fit not so good. Solar radiation and VPD are the main meteorological factors of the sap flow, there was a positive correlation between the sap flow and the solar radiation and the VPD in the form of a power function. The flow variation had a delay effect of 45-135 min to the solar radiation, but the sap flow peak duration was less than solar radiation peak about 1h. In the typical sunny day, the larger the DBH was, the greater the transpiration was, and the transpiration had a positive correlation with the DBH in the form of a power function. In the foggy days, the positive correlation between transpiration and DBH was not as good as sunny day.