The natural difference of stable isotope composition (δ¹³C) can accurately indicate the distribution of organic carbon in ecosystem and provide key information for the study of soil carbon dynamics. The characteristics of soil stable carbon isotope and its correlation with environment in arid and semi-arid areas were studied in order to provide the basis for revealing the dynamic changes of soil carbon in mountainous forests in arid and semi-arid areas. In order to understand the dynamics of soil carbon in mountain forests with different precipitation gradients, the soil samples of Picea crassifolia and Pinus tabulaeformis in Helan (HL), Luoshan (LS) and Liupan (LP) in Ningxia were selected as the research objects. The vertical and horizontal distribution of soil organic carbon (SOC), organic carbon δ¹³C and β value in 1 m soil were studied. Redundancy analysis was used to explore the effect of soil characteristics on soil organic carbon characteristics. Field investigation was carried out in Helan (HL), Luoshan (LS) and Liupan (LP) in Ningxia. In the range of 0～100 cm, and divided into 0～10 cm, 10～20 cm, 20～40 cm, 40～60 cm and 60～100 cm depth segments were selected and distributed in Picea crassifolia and Pinus tabulaeformis. In the horizontal direction, with the increase of precipitation, SOC content gradually increased, while δ¹³C and β values gradually decreased. In the vertical direction, SOC content and β value decreased with the increase of soil depth, while δ¹³C increased with the increase of soil depth. The results showed that the organic carbon content of the two coniferous forests in the three mountains was Picea crassifolia > Pinus tabulaeformis. The δ¹³C showed Picea crassifolia < Pinus tabulaeformis, and β value except Luoshan 0～10 cm was Picea crassifolia < Pinus tabulaeformis. In general, Picea crassifolia was more favorable to soil organic carbon accumulation than Pinus tabulaeformis. This study found that Available potassium (AK), available nitrogen (AN), available phosphorus (AP), total nitrogen (TN) and total phosphorus (TP) had small angle and consistent direction with organic carbon, showing significant positive correlation, and negative correlation with δ¹³C and β. Total salt (S) was negatively correlated with organic carbon, and positively correlated with δ¹³C and β. Total salt (S) had small angle and consistent direction with δ¹³C and β, showing a significant positive correlation. Among them, available nitrogen (AN) accounted for 34.7% of the differences in soil organic carbon characteristics (soil organic carbon content, δ¹³C and β values), respectively. Therefore, alkali available nitrogen was the main soil factor affecting the distribution characteristics of soil organic carbon. It is suggested that precipitation gradients, vegetation types and soil characteristics affected the distribution patterns of soil carbon in alpine ecosystems in arid and semi-arid areas. Soil available nitrogen is the key factor affecting the distribution pattern of organic carbon.