Abstract: Through tomato planted in barrels and plots in spring greenhouse, some experiments were carried out, which including the mechanism and law of tomato stem diameter variation, the influences of exterior environmental factors on its stem diameter variation, the way of eliminating meteorologic factors disturbances on observed Maximum Daily Shrinkage (MDS), in order to provide theoretical and practical basis for the technology of monitoring crop water content and automatic irrigation system based on crop stem diameter variation. The results showed that the stem diameter variation of tomato lagged the leaf water potential variation for 1–1.5 h, but they had better correlation. The shrinkage process of tomato stem was consisted of synchronous shrinkages of xylem and phloem, but the xylem recovered its size sooner than phloem did in its recovery process. The daily transpiration intensity in tomato fruit period was bigger than that in its flower and fruit period, which caused the daily minimum stem diameter in fruit period occurred latter than that in flower and fruit period, and the critical stomata conductance values of stem recovery in fruit period was smaller than that in flower and fruit period too. Furthermore, the variation of tomato MDS shared the same trend with the daily average radiation, but its values scopes were regulated by soil water content. With the decrease of soil water content from field water capacity (FWC) to 50% relative soil water content, and the value of MDS got bigger. The MDS could reveal the difference of soil water content obviously in sun day, but the value of MDS got smaller along with the decrease of soil water content on the contrary with the condition of relative soil water content below 50%. In addition, through statistical analysis, the RMDS which was the value of MDS/MDS※ could eliminate the disturbances of meteorologic factors on observed MDS and reflect soil water content stably.