Fine root decomposition can be affected by root order and soil depth. Using a branch-order classification, we separated the fine root systems of Larix gmelinii into two classes: first- and second-order roots were combined into one lower order (1-2 order); third- and fourth-order roots were combined into another higher order (3-4 order). Using the litterbag method, we conducted an 862-day decomposition experiment of lower-order and higher-order roots at different depths (0-10, 10-20, and 20-30 cm) to study decomposition and the nutrient dynamics of fine roots among different root branching orders. Our results showed that the decomposition rate of the 1-2 order was slower than that of the 3-4 order, and this pattern also held true at different soil depths. Spatially, the decomposition rate of both lower and higher roots decreased with increasing soil depth, and the decreasing trend in the higher roots was more obvious. With the process of decomposition, there were larger differences in the decomposition rates of each soil layer among the lower-order roots. The patterns of root nutrient release also varied depending on different stages of decomposition. In conclusion, the N release rate generally increased with root order, and decreased with increasing soil depth.