An improved fragmentation and diffusion (FD) coal pyrolysis model was developed based on the diffusivity correlation for the diffusion in fractal pores. Fitting functions to describe the continuous variation of pore structure during pyrolysis were obtained by fitting experimental data for coal pyrolysis in a drop-tube furnace. With the empirical functions as inputs, the improved FD model was used to simulate the pyrolysis in the drop-tube furnace and the model proved to be reliable. With this model, we found that the physical structure of char affects the diffusion, which then influences the pyrolysis. The effects of the four structural parameters (fractal dimension, porosity, average pore diameter, and specific surface area) are investigated quantitatively.