Abstract: The objective is to establish a constitutive model to describe the behavior of high-fiber-content functional gradation concrete (FGC). The model is based on Hooke’s law and linear elastic stress-strain relationship, introducing a fiber spacing coefficient along with a modification method for this coefficient to enhance the toughening effect of the fibers. The new correction factors include fiber orientation, fiber inclination, and fiber spacing coefficients, which improve the accuracy of predicting the mechanical behavior of FGC under conventional tensile or bending test conditions. This model addresses the shortcomings of existing constitutive models in the study of high-fiber-content materials. The model proposed is incorporated into ABAQUS to establish finite element models for FGC, including uniaxial tensile, dog-bone, and four-point bending models. The interaction between the FGC's upper and lower interfaces as well as between the fibers and the matrix is also considered. Results indicate that for FGC testing, the critical fiber volume fraction, ρ_s, ranges between 6% and 7%. When ρ_s is between 0% and 6%, the peak stress, tensile strength, and toughness significantly increase as the fiber volume fraction rises, without requiring consideration of the fiber spacing coefficient. When ρ_s exceeds 6%-7%, the rate of increase slows down, and peak stress and tensile strength may even decrease, necessitating the inclusion of the fiber spacing coefficient. A comparison with experimental results demonstrates that the model proposed is feasible for studying the mechanical properties of FGC. This model not only provides new theoretical insights for FGC but also has potential for application to other complex fibers or multilayered structures, offering significant academic value and engineering prospects. The critical value of ρ_s must be calculated for specific models, and additional testing and calculations are required to verify results for low-fiber-content steel fibers.