Adopting noise initiation mode of stimulated Brillion scattering (SBS), the influence of focal length on optical limiting power and energy is numerically simulated. It shows that the characteristic of limiting power and energy are controlled by focal length. When focal length is moderate, say 15cm, the limiting energy is lowest. When focal length is short, say 5cm, the characteristic of limiting power is close to ideal. In experiment, focusing 8ns, 1064nm, 16mJ pulses into CCl4 medium, the variation of limiting power and energy versus focal length is investigated. The experimental results are in good agreement with the conclusions predicted by the simulations.