Potassium is crucial in plant metabolism processes, and sufficient potassium can improve plant tolerance to abiotic stress. We studied the effects of different KCl concentration treatments (0, 1, 5, 20 mM) on the biomass, photosynthetic characteristics, and ion content of Zoysia matrella under salt stress (NaCl 300 mM). The results showed that the plant dry weight, stomatal conductance, transpiration rate, photosynthesis rate, K⁺ content in plants, and K⁺/Na⁺ ratio in leaves of Zoysia matrella under NaCl stress were significantly lower than those under no NaCl conditions. The addition of K can promote an increase in plant dry weight and significantly improve the stomatal conductance, transpiration rate, and photosynthesis rate of plants. In addition, under salt stress, the addition of 20 mM KCl can significantly reduce the accumulation of Na⁺ in plants and promote the secretion of Na⁺ in leaves, thus improving the salt tolerance of Zoysia matrella.