Growth of spatial nonuniformity of laser beam propagating in the near-resonant atomic vapor was numerically studied for atomic vapor laser isotope separation (AVLIS). The simulation results well reproduced the linear and nonlinear growth and the beam breakup. The growing region of nonuniformity and the maximum growth rate were analyzed in relation to the spatial wavenumber of ripple and the nonlinear refractive index of the near-resonant atomic vapor. And the effects of ripple growth on AVLIS were discussed.