The invention provides a method for modifying cotton and long vegetable fiber or yarn by ammonia liquid, which comprises the following steps: firstly, drying pre-treatment of raw materials; secondly, ammonia leaching of the raw materials in a hermetical treatment tank; thirdly, spin-drying of the raw materials subjected to ammonia leaching when a creel is twirled; fourthly, drying of the raw materials subjected to spin-drying by ammonia flow when the creel rotates slowly; fifthly, flushing of the raw materials which are dried by the ammonia flow by airflow when the creel rotates slowly; and so on. The method keeps the inherent advantages of the cotton, the long vegetable fiber and the yarn, overcomes the defects of the cotton, the long vegetable fiber and the yarn, increases softness, crimp tendency and fluffiness of the cotton, the long vegetable fiber and the yarn, improves the dye-uptake rate, the leveling property and the showy dye level, and further solves the problems of energy conservation, efficiency improvement and environmental protection.