The primary forest of Abies georgei var. smithii was taken as the research object，and the soil nutrient content of different decomposition levels and the underlying soil were sampled，and the dynamics of understory nutrient and its effects on soil nutrient were analyzed. The results showed that:(1)With the deepening of decomposition degree of fallen wood，the contents of total organic carbon，total potassium，cellulose and pH value in the underlying soil decreased，while the contents of total phosphorus，calcium，magnesium and total nitrogen increased，and the changes of sodium and lignin were not obvious；(2)The contents of soil organic matter，available potassium，total nitrogen and available nitrogen decreased significantly with the deepening of the soil layer under fallen wood，while the contents of total potassium，total phosphorus and available phosphorus had no significant change with the deepening of the soil layer under fallen wood，while the soil pH value showed an increasing trend. In conclusion，the decomposition of fallen fir forest in Sejila mountain plays an important role in the maintenance of soil nutrients. It is suggested that the management and protection of fallen fir resources in forest should be further strengthened in future forest management.