Gas effective diffusivity of single coal char particle is a key parameter in modeling coal-fired traveling grate boilers, which are extensively used in China with a low efficiency. The effective diffusivity varies during the combustion because of the changing pore structure caused by ash layer development. A macro-TG was designed to obtain the evolving effective diffusivity by measuring the combustion rate of a single coal char particle in one-dimensional combustion. The effective diffusivity decreased with the layer thickness increase, and both coal rank and temperature were contributing factors to the variation behavior. An exponential function was found between the effective diffusivity and the layer thickness, and was used to reveal the pattern of pore structure evolution.