Corn straw was added to low, medium, high, and removal organic soil for indoor culture test to study the influence of straw addition amount on soil acidity and buffer performance of soil with different organic matter contents. The results showed that after 150 days of constant temperature culture at 15 ℃, the contents of soil pH value, exchangeable salt-base, CEC, and salt-base saturation all increased with the increase of straw addition, while the contents of exchangeable H⁺ and exchangeable Al³⁺ decreased with the increase of straw addition. Among them, when the addition amount of straw was between 10 g/kg and 15 g/kg, the increase of soil pH value and the content reduction of exchangeable Al³⁺ and exchangeable H⁺ were the largest in the LSOM treatment. When the adding amount of straw was between 15 g/kg and 20 g/kg, the increase of soil pH value and the content of soil exchangeability Al³⁺ and exchangeability H⁺ decreased the most in the MSOM treatment and HSOM treatment. Moreover, adding straw to the HSOM treatment had the best effect on reducing soil acidity. Adding straw to the soil can obviously improve the buffer capacity of soil acid-base, and the buffer capacity of soil to the alkali side was more obvious than that to the acid side. The buffer capacity of soil increased with the increase of straw addition, and the acid and alkali buffer capacity of treatment was HSOM > MSOM > LSOM > RSOM.