Abstract: To ascertain the variation law and effect factors of sap flow rate, dynagage sap flow sensors (developed by Dynamax Inc.) were used to measure the sap flow rate of tomatoes in greenhouse, and the variation law and standardized processing technology of sap flow rate were studied in this paper. The relationship between sap flow rate and meteorological factors was discussed, and the effects of water stress on sap flow rate of tomato were also analysed. The results showed that using sap flow rate per unit leaf area to express the variation of sap flow rate could reduce the difference in sap flow rates among various plants. Solar radiation and vapor pressure deficiency were the main factors affecting the sap flow rate of tomato under adequate water-supply conditions, in which there were linear relationship between daily sap flow rate and sun radiation, and logarithmic relationship between daily sap flow rate and vapour pressure deficit (R2>0.90, P<0.01). Sap flow rate of tomato was affected by soil water moisture. Sap flow rate decreased with soil water stress. Research results show that standardized sap flow rate of tomato can reflect the true transpiration.