The gain and excess noise of multi-gain-stage superlattice InGaAs APD was mainly studied in this paper, and a new multiplication-excess noise model of carriers was established. Based on the conventional Dead Space Multiplication Theory, we analyzed its working principle. Additionally, we considered initial energy from pre-heat electric field and energy band offset, and the modification of dead space length around heterojunction′s boundary when carriers entered high-field multiplication layer, as well as the effect of phonon scattering on impact ionization coefficients. Thus we proposed a modified Dead Space Multiplication Theory to guide the calculation of the gain and excess noise factor of this type of APD device. The results demonstrated that under the same condition, multi-gain-stage superlattice InGaAs APD has both higher gain and lower noise than conventional SAGCM APD with a single multiplication layer, and the modified Dead Space Multiplication Theory can be extended to superlattice InGaAs APD structure with more gain stages. On the premise of low excess noise, its mean gain can be improved by increasing number of gain stages.