In this talk, I will first introduce our recently developed numerical method to study the atomic ionization process in the intense mid-infrared laser field. Then, I will present a systematical study of atomic Xe ionization in intense laser field with several laser wavelengths up to 2000 nm, both theoretically and experimentally. A simple model --interference between an plane wave and spherical wave-- is introduced to explain the observed momentum distribution. We attribute the lower energy structures to the multiple-scattering processes.