A two-dimensional axisymmetrical mathematical model for the isothermal chemical vapor infiltration process of C/SiC composites was developed. Transport phenomena of momentum, energy, and mass in conjunction with infiltration-induced changes of preform structure were taken into account. The integrated model was implemented by the finite-element method to simulate numerically the isothermal chemical vapor infiltration (ICVI) process of C/SiC composites at different methyltrichlorosilane (MTS) fluxes. The influence of MTS flux on concentration distribution and time-dependent densification behaviors of C/SiC composites was studied in detail. Calculation results imply that MTS flux has an obvious influence on infiltration in micro-pores and little influence on infiltration in macro-pores. Increasing flux will lead to an evident acceleration for infiltration in micro-pores. Moderate flux is preferable by a combination of both a relatively high infiltration rate and a relatively low fabrication cost. This model is helpful to understand the fundamentals of the ICVI process for the fabrication of C/SiC composites.