Sap flow is the source of water for plant evapotranspiration, which is the direct quantitative index of plant water consumption. We monitored the sap flow of Lycium barbarum continuously with stem heat and balance sap flow monitoring techniques for the entire growing season in an arid region of Ningxia. We used these data to analyze the variation in sap flow and determine its relationship with meteorological factors on different time scales. Our goal was to provide evidence to clarify the amount of water consumption by Lycium barbarum and the factors that influenced water consumption. The results indicated the occurrence of diurnal variation in sap flow. Sap flow was approximately 10 times greater during the day than at night. Sap flow started around 06:30 and occurred approximately 30 min earlier on cloudy days. Additionally, the sap flow rate curve had a broad peak on sunny days. In summer, sap flow started at 06:00, 1 hour earlier than in autumn, and the maximum sap flow rate was 123 g/h. The sap flow rate was greatest during the full fruiting period and had a minimum value during the vegetative period. It increased rapidly from June to August when water consumption was 1388.3 g/d. Average daily water consumption was 1102.7 g/d during the entire growth period. We also found that the sap flow rate was positively correlated with solar radiation and air temperature, whereas a negative correlation with relative humidity was detected. Furthermore, sap flow (F) expresses the following quantitative relationship with solar radiation (S), temperature (T), relative humidity (H), and vapor pressure differential (VPD):F=41.5+0.167S-0.563H+1.36T-9.67VPD (R²=0.6547).