Micromagnetic modeling, as a progressive numerical simulation method of magnetic research, can reveal the magnetization mechanism of materials while saving experimental costs. However, its application scale is limited by the enormous computational cost. Currently, micromagnetic simulations are mainly applied to calculate the magnetization of nanocrystalline magnets below the micrometer level. For larger magnets with grain sizes in the range of tens micrometers, such as sintered Nd-Fe-B magnets, this high-precision numerical calculation method may be less effective. To end this, this study proposes a nanocrystal-sized micromagnetic model, which is adjusted by fitting the magnetization relaxation time and intergranular coupling strength. Through incorporating time and spatial correction coefficients, it can calculate the magnetization behavior of large-grain magnets effectively. The thermal demagnetization process of sintered Nd-Fe-B magnets is further analyzed, and the micromagnetic simulation results are compared with the experimental results from temperature-controlled demagnetization experiments, which confirms the accuracy of the equivalent method. This research expands the research application scale of micromagnetic simulations and reveals the magnetization mechanism of sintered Nd-Fe-B magnets under thermal effects.