As the nighttime replenishment engine, root pressure is one of the most important plant water physiological indices. But root pressure measurement is still a technical problem needed to be resolved. Especially for little plants or herbaceous plants, they are not allowed to be measured invasively. Thus it is a greater technical challenge. On the basis of the combined measurement of stem diameter and sap flow, we attempted to measure the plant root pressure by changing the algorithm orders of the Steppe water flow and storage model. Three greenhouse eggplant samples were selected in the experiment and two groups of five consecutive days of experimental data, which were independent, were displayed. The results showed that from night to dawn, the sap flow rate was zero and the stem grew slowly. Meanwhile, the root pressure began to appear. In the sunny days, the transpirations of eggplant samples were more intensive, the sap flow rates were higher and the stem shrank more significantly. Accordingly, the root pressure increased faster at night. On the contrary, when it was cloudy, the root pressure amplitudes of the eggplant samples were smaller. Obviously，the dynamics of the root pressure were fully consistent with the interpretation for the dynamics of the sap flow rate and the stem diameter,the influence of meteorological data and the known plant physiological water adaptability law. Therefore, it’s feasible to observe the root pressure dynamics of the eggplant samples nondestructively by changing the algorithm orders of the Steppe water flow and storage mathematical model. It allows conducting the similar experiments for other plants.