Packed bed reactors have been widely used in chemical reaction engineering. A kind of grille-sphere composite packed bed has been proposed to decrease the pressure drop and increase the radial heat transfer performance. In the present paper, both the homogeneous model and the solid particle method are used to study methane steam reforming reactions in a grille-sphere composite packed bed. Firstly, the homogeneous model is adopted to compare the overall chemical reaction performance between the randomly packed bed and the grille-sphere composite packed bed. In the homogeneous model, the packed bed is considered as a porous media and the kinetic reaction rates are incorporated into Fluent software with user-defined functions. Secondly, an optimization is conducted by using the solid particle method for packing structure in the single channel of the grille sphere composite packed bed with sub-channel to particle diameter ratios between 1.00 and 2.00. In the solid particle method, both fluid and solid domains are included. User-defined scalars are used to represent species inside catalytic particles and the chemical reactions are modeled with user-defined functions. It is found that when compared with the randomly packed bed, the conversion rate of methane in the grille-sphere composite packed bed is increased from 9.58% to 12.30% and the temperature of the tube wall is lowered by 50 K, indicating a longer tube life. Moreover, in the grille-sphere composite packed bed, with the proper selection of sub channel to particle diameter ratio and the packing form, the pressure drop is greatly reduced and the heat of the chemical reaction is improved. When sub-channel to particle diameter ratio is less than 1.71, the main stream is not disturbed by particles, which results in lower pressure drop and higher overall thermal storage performance.