Alfalfa (Medicago sativa L.) is a high quality forage that is widely planted in arid and semi-arid regions of the world. Although it has high drought resistance capability, it still requires much water for growth and development. Soil water supply as one of the main factors limiting the expansion of alfalfa cultivation could severely affect its productivity. The changes in precipitation in some regions as a result of global climate change, such as exacerbation of drought, can result in the reduction of alfalfa yield and quality. Therefore, it is important to study the drought resistance mechanisms in alfalfa. Leaf is one of the most important organs linking a plant with the surrounding environment and is one of the most sensitive parts in the plant responding to changes in environmental factors. Leaf traits evolved as a result of balance (trade-off) between growth and survival (resistance and tolerance) in adverse environment, i.e., water deficient environment. However, a systematical investigation on the responses of leaf traits to drought is still lacking, and the changes in leaf traits have rarely been investigated at different plant growth stages. In addition, most of the previous studies were primarily on trees and less on herbaceous species, such as alfalfa, which differ heavily in morphological traits from trees. Therefore, in this study, a pot experiment was performed to investigate the responses of leaf traits ofM. sativa ‘Longdong’ to drought at different growth stages. The aim of the study was to test the hypothesis that leaf traits in alfalfa change in different growth stages in response to water deficit. The results are as follows: (1) With the increase in soil drought, leaf dry matter content (LDMC), leaf nitrogen concentration per unit mass (LN), leaf mass per unit area (LMA), and leaf thickness (LT) increased, leaf area and leaf mass per unit area were sensitive to soil water deficit, whereas leaf area (LA) decreased, leaf phosphorus concentration per unit mass (LP) remained unchanged, and leaf length/width ratio (L/W) and leaf chlorophyll content (Chl) varied wildly without showing clear trends. (2) As the plants progressed through growth stages, LT, LDMC, L/W, and LA did not exhibit appreciable changes, whereas LN decreased, LP increased and then decreased, and LMA and Chl showed large fluctuations. (3) Pearson correlation analysis showed that L/W was negatively correlated with LN and LT, LDMC had positive correlation with LT and LMA. LMA was positively correlated with LT and LN and negatively correlated with L/W and LP. LN had a positive correlation with LT, and LA was positively correlated with LP and negatively correlated with LDMC, LMA, and LN. There was no significant correlation between Chl and other traits. Although there was no definite trend in changes in leaf traits to drought at different growth stages, the variation of leaf traits at different stages reflects a fundamental strategy of alfalfa's response to drought. To the best of our knowledge, this is the first study to elucidate the adaptation of alfalfa to drought through investigating changes in leaf traits at different growth stages.