The terrestrial ecosystem carbon cycle, an important component of the global carbon cycle, has significant impacts on the composition of atmospheric greenhouse gases (GHGs) and global climate change. Quantitative analysis, which provides knowledge of the terrestrial ecosystem carbon cycle, is not only a prerequisite for accurately predicting regional and global climate; it also provides a scientific basis from which governments can develop programs in response to global change. The major goal of this study is to quantify the carbon cycle of the Larix chinensis forest at Taibai Mountain, Shaanxi, China, which experienced significant climate change during the time period from 1958 to 2008. Process-based models are useful tools for predicting changes to the long-term ecosystem carbon cycle that is influenced by climate change. We quantitatively investigated the carbon cycle of the modeled area by using a process-based model, LPJ-GUESS, using climatic data from the Mountain Microclimate Simulation Model (MTCLIM) and CO₂ concentration data to drive the model. Net primary productivity (NPP), biomass and net ecosystem carbon exchange (NEE) are the major outputs of the LPJ-GUESS model we used in this study. The NPP and biomass of L. chinensis and Abies fargesii were large in the L. chinensis forest, and both NPP and biomass of L. chinensis were larger than A. fargesii. The average NPP of L. chinensis and A. fargesii during 1958 and 2008 were 0.38 kgC·m^-2·a^-1 and 0.25 kgC·m^-2·a^-1, respectively. The sum of the two species' NPP accounted for 86% of the biomass in L. chinensis forest. The average biomass of L. chinensis and A. fargesii during 1958 and 2008 were 2.91 kgC/m² and 2.02 kgC/m², respectively. The sum of the two species' biomass was 94 percent of the NPP in L. chinensis forest. A. fargesii occupied the lower slope of the L. chinensis forest because of its altitudinal limitation, and the upper elevation forest was pure L. chinensis forest. Both L. chinensis and A. fargesii had larger NPP on the northern slope of Taibai Mountain than on the southern slope. Both slopes experienced increasing NPP of the two species over time, and the increase of the northern slope was smaller than the southern slope, so the difference of the L. chinensis forest's NPP between the northern and southern slope was decreasing over the 51-year study period. Biomass of L. chinensis experienced inter-annual fluctuations during the 1958-2008 study period and increased alternatively between the northern and southern slope. The mean biomass on the southern slope between 1958 and 2008 (2.94 kgC/m²) was greater than the northern slope (2.89 kgC/m²). Biomass of A. fargesii, which always occupied a larger part on the northern slope during the 51-year study period, had fewer inter-annual fluctuations compared with L. chinensis. The modeled value of NEE, whose average was -0.023 kgC·m^-2·a^-1, showed that L. chinensis forest has been a carbon sink over the 51 years studied. There was no significant difference in average NEE between the northern and southern slope. Both slopes showed increasing carbon sink functioning, with an annual increase of 0.91 gC·m^-2·a^-1 on the southern slope and 0.42 gC·m^-2·a^-1 on the northern slope. This long-term simulation of the L. chinensis forest driven by climatic data and CO₂ concentration was only a preliminary quantitative study of the carbon cycle dynamics. Additional field observations and manipulative experiments are needed to reveal the relationships between climate change and the ecosystem carbon cycle.