Abstract: Based on scale analysis and order estimation, the inertia-gravity waves and vortical flows of barotropic geophysical fluid are investigated in the adjustment and evolution processes, and the contribution of various factors to the ageostrophic modes and the source of inertia-gravity waves in the two different dynamic stages of geostrophic adjustment and quasi-geostrophic evolution are discussed. For the linear small perturbation case, if the \beta effect is not considered, the small perturbations in the f plane establish the steady geostrophic balance through the adjustment process, and the steady-state of geostrophic balance will be maintained, and no more ageostrophic modes of motion, such as inertia-gravity waves, will be excited. Considering the \beta effect, the motion on the \beta plane transitions the stage of quasi-geostrophic evolution process after the adjustment process, and the unsteady evolution process will cause fluctuations of the geopotential field, which in turn generate the ageostrophic perturbations and excite the new inertia-gravity waves. In the full nonlinear case, excluding \beta effect, in the evolution stage, under the constraint of conservation of potential vorticity, the unsteady quasi-geotrophic flows generate new ageostrophic modes of motion by altering the divergence of geopotential flux due to the generalized potential vorticity advection, thereby excites new inertia-gravity waves. For the Burger number Bu \ll 1, i.e., the spatial scale L is much greater than L_d, the Rossby radius of deformation, the effects of both the \beta effect and the advection of generalized potential vorticity have to be accounted for, which generate ageostrophic modes of motion by altering the potential flux divergence within the constraints of potential vorticity conservation, thereby exciting inertia-gravity waves.