Abstract: Generally, the changes of the leaf potential and the transpiration almost depend on the characteristics of biology, and also are constrained by environmenal factors. Thereby, according to the data of the field tests, the daily variation law of leaf potential and the transpiration was analyzed under the different levels of irrigation. The relationships among leaf potential, transpiration rate and soil water content, meteorology factors(including temperature, net radiation, saturation deficiency) were analyzed. And the quantity formula of leaf potential, the transpiration and environmental factors was established. The study has reference significance for instructing reasonable irrigation of alfalfa.