Abstract: To see the relationship between the structure of a combustor and the flame length in diffusion combustion,variation of the flame length with spouts space is numerically researched based on the existing work.Result shows that if velocity of gas is unvaried,the length of flame will periodically change with the space between the spouts,which is from 0.5 to 2 times diameter of the spouts.The period is about 0.5 times diameter of the spouts.The velocity of co-flow air is not effective on periodicity of the length of flame that changes with the spouts space.