Using a linear stability analysis method,a linearizing model derived from the nonlinear schroedinger equation is used to analyze the influence of the air gap between slabs on small-scale selffocusing modulation growth in a slab amplifier of a high power laser facility. The quantitative relations between B-integral,the number of slabs,the ratio of air gag interval distance to the thickness of slab,and nonlinear modulation gain spectrum are given explicitly. It is found that as the gap distance increase,the nonlinear gain spectrum moved toward to long spatial wavelength region. The number of slabs also influence the maximum gain value and its spatial frequency. In addition,an approximate expression was given to show the air gap influence by using a gain spectrum without air gap but with an equivalent power and B-integral value.