Stem water storage varies with a daily cycle. However, none of sensor techniques can meet the noninvasive measurement of herbaceous plant stem water storage. A method of using a sap flow rate sensor to observe stem water storage dynamics for herbaceous plants was proposed. The preliminary experiment was conducted with a potted sunflower. The sunflower stem internal water recharging/discharging and its switching time could be obtained by the outputs of a sap flow sensor and an electronic scale. The observation of sunflower stem diameter variations further validated the effectiveness of the method. At the same time, the micro meteorological factors could be collected and the results showed that the micro meteorological factors had a great impact on stem water storage dynamic changes. Compared with other methods, the measurement system is simple, easy to operate and non destructive. Even though only sunflower samples were investigated, the method can be also applied to other herbaceous plants.