Abstract: By taking Chinese fir plantations with different forest ages (5, 8, 21, 27, 40 years old) in subtropical region as the research object, the differences of soil nitrogen mineralization rate, the contents of microbial biomass nitrogen and available nitrogen in Chinese fir plantations with different forest ages and the key influencing factors were explored. The results showed that the content of soil mineral nitrogen decreased with the increase of forest age, and the contents of microbial biomass nitrogen and mineral nitrogen in the 27-year-old Chinese fir plantation were significantly higher than those in the 21-year-old Chinese fir plantation. The soil net nitrogen mineralization rate fluctuated with the forest age, among which the soil net nitrogen mineralization rate in the 40-year-old Chinese fir plantation was significantly higher than those in the 8-year-old and 27-year-old Chinese fir plantation. The soil net nitrogen mineralization rate was positively correlated with the nitrate nitrogen content, and had significant negative correlation with the contents of total nitrogen and total phosphorus. The results showed that the application of phosphorus fertilizer in the forest culture and management stage and the adoption of thinning measures could reduce the soil nitrogen loss to a large extent.