Abstract: The effect and mechanism of calcium on plant growth and potassium uptake were analyzed under low potassium conditions. Plants of tobacco variety K326 were grown under indoor hydroponic conditions, and two potassium levels and six calcium concentrations were set to determine physiological characteristics of plants, root calcium ion concentration, root ATPase activity and potassium concentration in various parts of the plant. The results showed that:Different potassium levels significantly affected tobacco plants' growth and potassium uptake. Dry and fresh mass of shoots and roots, physiological characteristics and potassium content in various parts of the plants under low potassium conditions were lower than those under normal potassium conditions. Potassium contents in shoots and roots of tobacco plants under low potassium condition were decreased by 71.80%-25.63% and 88.68%-45.47%, respectively. At the same potassium level, dry and fresh mass of shoots and roots, physiological characteristics, and potassium contents in various parts of the plants showed a trend of increasing firstly and then decreasing with the increase of calcium concentration. Under low potassium conditions, dry and fresh mass of each part of the plants, soluble protein content in leaves and roots, POD and CAT activities in leaves, potassium content in every parts of tobacco plants were optimal if provided with 5 mmol/L calcium concentration. Root potassium content reached 7.323 mg/g. Low potassium condition significantly affected tobacco plant growth and potassium uptake. Appropriate Ca²⁺ concentrations can promote K⁺ uptake, but excessive Ca²⁺ can inhibit K⁺ uptake. Calcium concentration of 5 mmol/Lunder hydroponics is the most suitable for tobacco growth, development and potassium uptake.