In order to comprehensively understand the patterns of changes in the chemical traits of plant fine roots after nutrient addition, the study selected 8 woody plants and 6 herbaceous plants; nitrogen and phosphorus were separately applied to their pots. In September 2017, we measured carbon and nitrogen chemical traits of different diameter classes (with root diameters of less than 0.5 mm; root diameters of 0.5-1 mm; root diameters of 1-2 mm). It analyzed the impact of nitrogen and phosphorus addition on the total root carbon concentration (RTC), total root nitrogen concentration (RTN), and carbon-tonitrogen ratio (RC/N) in plant fine roots. The research indicated that the impact level of fertilizers and the interaction between species and fertilizers on the RTN and RC/N are highly significant. Under N addition, all species showed an increase in the RTN across different diameter classes and a decrease in RC/N; however, it is inconsistent under P addition. Herbaceous and woody plants exhibit a significant increase in RTN across different diameter classes after nitrogen fertilization, while woody plants show a significant decrease in RC/N. This suggests that under nitrogen addition, woody and herbaceous plants exhibit similar trends in the RTN across different diameter classes. Regardless of fertilization, there is a significant negative correlation between the RTN and RC/N for each diameter class. The relationship between RTC and RC/N is not clear, indicating that although fertilization alters C and N chemical traits, the relationship between them does not undergo a significant change.