This study aimed to explore the adaptability and mechanism of walnut seedlings under potassium (K) deficiency stress. Potassium is a major nutrient element required for plant growth and development and has an important impact on plant physiological processes. Understanding the response mechanism of walnut seedlings under potassium deficiency conditions is of great significance for improving crop stress resistance. In this study, different potassium deficiency treatments were designed, including the control group (CK), moderate potassium deficiency (MK), and severe potassium deficiency (SP), to explore the effects of potassium deficiency on the growth and physiological characteristics of walnut seedlings. The effects of potassium deficiency on seedlings were evaluated by observing changes in plant growth indicators, photosynthesis, hormone content, etc. Potassium deficiency significantly affected the growth and physiological characteristics of walnut seedlings. With the increase in potassium deficiency, the seedlings’ aboveground biomass, root biomass, and chlorophyll content decreased. At the same time, the root activity and specific root area of the severe potassium deficiency group were significantly higher than those of the control group. The photosynthesis rate and stomatal conductance were the highest in the control group and gradually decreased in the potassium deficiency group. Hormone analysis showed that potassium deficiency treatment led to increased levels of hormones such as IAA, CTK, and ABA, and dynamic changes occurred with the extension of treatment time. Walnut seedlings relieve physiological stress by regulating hormone and polyamine content under potassium deficiency stress. However, long-term potassium deficiency interferes with hormone synthesis and plant growth and development, which may eventually lead to plant senescence and growth stunting.