There is a serious effect directly for optical element sub-surface damage to optical performance and laser damage threshold in the application, and the primary cause is due to light scattered by subsurface damage. Aim to K9 glass optical element, the simulation model is built. By using finite difference time domain (FDTD) method, the light field distributions, which are the incident light modulated by subsurface micro-cracks and bubbles, are simulated, and the scattering light intensity variation received by detector is analyzed. The results have important theoretical guideline to further study the measuring methods of optical components subsurface damage.