Unreasonable cultivation is one of the main causes of soil degradation. In order to reveal the effect of cultivation on the change characteristics of soil organic nitrogen in alpine grassland, this study took alpine meadows of northwestern Sichuan after cultivation for 1, 3, 10, 16, 27 and 40 years as research objects and the uncultivated natural grassland as the control. The effects of cultivation on total acid hydrolysable nitrogen (TAHN), acid solution of ammonium nitrogen (ASAN), amino sugar nitrogen (ASN), amino acid nitrogen (AAN), acid hydrolysable unknown nitrogen (AHUN) and acid insoluble nitrogen (NAHN) were studied through soil sampling and analysis. The results showed that the contents of TAHN, ASAN, ASN, AAN, AHUN and NAHN decreased significantly (P < 0.05) with the increasing of cultivation years in the alpine grassland in northwestern Sichuan, especially in 0—20 cm soil layer, which decreased by 72.15%, 62.72%, 66.08%, 63.44%, 94.00% and 51.78%, respectively. The AHUN was the most significant decrease among the organic nitrogen components. The decrease of TAHN, ASAN, ASN, AAN, AHUN and NAHN mainly occurred in the first ten years of cultivation, and the annual average reduce rate decreased with the increasing of reclamation years. Therefore, reducing the reclamation of alpine grassland is important to promote the ecosystem balance and sustainable development of alpine grassland in the northwest Sichuan Plateau.