The output beam quality of high-power laser systems is limited by laser-induced thermal aberration of fused silica reflective optics. A numerical model for the simulation of thermal aberration was proposed and verified by the experimental results. Simulations on the thermal aberration of fused silica optics under 3~10 kW laser irradiation with laser beam diameters of 5 mm ~ 45 mm were carried out with the verified model. The simulation results showed that the peak-valley (PV) value of thermal aberration increases with increasing incident laser power under the same incident laser spot size and reduces with increasing incident laser spot size under the same incident laser power. There are the same PV values of thermal aberration under different incident power or power densities. An analytic formula of thermal aberration PV as a function of incident laser power and beam spot size was proposed. The analytic results are in good agreement with the simulations. With these conclusions, the thermal aberration of fused silica optics under high incident power and power density can be evaluated by that under low incident power and power density. It is helpful for the design of high-power laser systems to obtain reasonable output beam quality.