According to the properties of the small-scale self-focusing of the multi-photon nonlinear Compton scattering in gain medium,the theory of the nonlinear scattering of the small-scale self-focusing was put forward,and the small-scale modulation equation,gain relation and filamentation distance were revealed. The results show that the critical frequency and the fastest growing frequency are almost the same. In the gain medium,the fastest growing frequencies of the nonlinear disturbance and the largest rate of rise taken place by Compton scattering almost have not relations with the background optical intensity and propagation distance. Under the certain input and output powers,in the initial stage of the scattering,the background optical energy,the fastest growing frequency of the small-scale disturbance,the largest gain and its B integral are quickly increased,the scope of the modulating frequency is quickly widened,and the filamentation distance is prolonged by the scattering optical. Then,their changes almost have not relations with the scattering.