Plant roots are an important part of global terrestrial ecosystem carbon storage and play an important role in global ecosystem carbon cycle. Increasing nitrogen deposition will affect the allocation of root biomass in different soil layers and diameter classes, and further affect the productivity of forest ecosystem and soil nutrient cycle. In this study, the effects of four-year nitrogen deposition on the root biomass of Chinese fir saplings in different soil layers and different diameter classes were investigated. The results showed that: (1) Fine root biomass in the low nitrogen (LN) and the high nitrogen (HN) treatments did not differ from that of the control (CT) (P > 0.05). The coarse roots and total root biomass in HN treatment were 45% and 40% higher than that of CT, respectively (P < 0.05). (2) The biomass of fine roots and coarse roots in 20-40 cm and 40-60 cm soil layers increased significantly after nitrogen addition. In addition, the proportions of fine root biomass and coarse root biomass increased in the 20-40 cm soil layer in LN treatment; (3) Compared with the CT, HN treatment significantly increased root biomass of 2-5 mm, 5-10 mm and 10-20 mm diameter classes, LN treatment significantly increased root biomass (2-5 mm and 5-10 mm in diameter), but reduced root biomass (20-50 mm in diameter). Overall, after nitrogen deposition, Chinese fir saplings could increase the transport of nutrients and water by increasing roots with larger diameter, and maintain the rapid growth by increasing the biomass and proportion of roots in deeper soil layer. To a certain extent, the increase of root biomass will increase carbon input through roots and affect the process of soil carbon cycle.