Abstract: Coniferous and broad-leaved mixed forests in Southwest of Zhejiang Province are jointly affected by the mid-subtropical monsoon climate and complex mountainous topography, resulting in pronounced seasonal fluctuation in ecosystem carbon exchange. Clarifying the dynamic characteristics of carbon flux and its environmental responses is helpful for understanding the formation process of carbon sinks in subtropical mountain forests and provides a basis for regional forest carbon sink assessment and management. Based on the eddy covariance observational data from a 48-m flux tower in 2024, this study analyzed the seasonal dynamics of net ecosystem exchange (NEE), gross primary productivity (GPP) and ecosystem respiration (Reco), as well as their relationships with environmental factors in this typical coniferous and broad-leaved mixed forest. The results showed that: (1) in 2024, the ecosystem of the study area wholly functioned as carbon sink, with NEE being predominantly negative throughout the year. NEE ranged from −8.39 to 6.83 g·m⁻²·d⁻¹ at the daily scale, with an annual mean value of −1.38 g·m⁻²·d⁻¹. May to August was a period of relatively strong net carbon uptake, and the monthly mean NEE was lowest in July, at −3.75 g·m⁻²·d⁻¹. (2) Both GPP and Reco showed obvious seasonal variation, generally exhibiting a trend of being lower in winter, increasing in spring, higher in summer, and declining in autumn and winter. The intra-annual fluctuation of NEE was closely related to the relative changes in GPP and Reco. (3) Correlation analysis showed that GPP was highly significantly positively correlated with air temperature (T_a), soil temperature (T_s), total radiation (Rg) and vapor pressure deficit (VPD) (P<0.01); Reco was mainly significantly positively correlated with Ta and Ts (P<0.05); NEE was highly significantly negatively correlated with GPP and Rg (P<0.01), significantly negatively correlated with VPD (P<0.05), and highly significantly positively correlated with Reco (P<0.01). T_a, T_s, Rg and VPD were environmental factors closely related to the intra-annual variation of carbon flux in the study area. The results can provide continuous observational evidence for assessing the carbon sink function of subtropical coniferous and broad-leaved mixed forests in southwestern Zhejiang and for regional forest management.