Experimental and numerical studies were conducted to determine the combustion characteristics of gas diffusion combustion in a porous combustor packed with 2.5 mm or 3.5 mm alumina pellets，special attention is focused on the effect of packed bed height (h) on combustion, NO and CO emissions. The pollutant emission of diffusion filtration combustion is studied with different packed bed lengthsin the range of 40 mm≤h≤240 mm, fixed excess air ratio of 1.88 and fixed gas inlet velocity of 0.06 m/s. Results show that both immerged and surface flames coexist in the combustor. Although porous media enhances the mixing and diffusion processes, the diffusion flame shape is still observed from the side and top views of the combustor, the diffusion filtration remains properties of diffusion combustion. The immersed flame is always observed with increase in h, whereas the height of surface flame decreases. The NO emission decreases sharply when his increased from 40 mm to 120 mm. However, the NO emission decreases slightly when h>120 mm. In the investigated range of h, it is shown that h has a significant influence on the CO emission, an increase in h leads to a constant increase in CO for the combustors packed with 2.5 mm or 3.5 mm pellets.The maximum CO emission is 662 ppm and the minimum value is 67 ppm. In the scope of this study, the temperature on the external wall of the combustor reaches 434℃-513℃.