[Objective] This study aims to investigate the carbon storage, carbon sink function, and response mechanism to climate in Pinus sylvestris var. mongolica plantations with different initial planting densities on the southern edge of Horqin Sandy Land. The goal is to facilitate the assessment of the carbon sequestration function and adaptive management of forest ecosystems.[Methods] Estimating carbon storage and carbon sequestration rate of Pinus sylvestris plantations using stand height and cross-sectional area, and analyzing their responses to temperature, precipitation, and evaporation in conjunction with meteorological factors. The GM (1,1) grey prediction model was used to predict forest carbon storage in 2030. [Results] The carbon storage and sequestration rate of Pinus sylvestris plantations with different initial planting densities exhibited similar annual fluctuations, with an overall increase in carbon storage each year. The curve of the sequestration rate showed a "U" shape. Both thinly and excessive initial planting densities can reduce the carbon carbon sequestration capacity of Pinus sylvestris plantations. Before reaching 32 years old, the highest carbon sequestration intensity was observed in stands with initial planting densities of 1 500~2 000 tree/hm², and for stands aged 35~46 years, the optimal density was 1 000~1 200 tree/hm². The carbon storage increased logarithmically with increasing stand density.The response pattern of the planted Pinus sylvestris forests’carbon sequestration rate differed among stands with different initial planting densities regarding temperature but not precipitation. For high and extremely high-density Pinus sylvestris plantations, the average temperature in August of the previous year and in March, May, June and July of the current year were the main climatic factors limiting carbon sink. For low and medium-density plantations, the average temperature in August of the previous year and in March and October of the current year were the main climatic factors that constrain carbon sink. The correlation analysis between evapotranspiration and carbon sequestration rate in different planting density stands showed that carbon sink of low-density Pinus sylvestris plantations was more sensitive to evapotranspiration. Based on the GM (1,1) grey model, it was found that excessively high or low initial planting density would reduce the carbon sequestration potential of future Pinus sylvestris plantations. The optimal planting density for carbon sequestration rate was 1 772 tree/hm². [Conclusion] The initial planting density of Pinus sylvestris has a significant impact on carbon storage and carbon sequestration, as well as their response to climate change in sandy areas. Adjusting the initial planting density may be one of the key adaptive management measures for Pinus sylvestris plantations under climate change.