Abstract: Mirror flatness greatly affects beam quality of output laser in laser system. In order to quantitatively evaluate influence of mirror defects on laser beam quality, parameters τ of describing relevance between mirror surface defects and laser beam, and δ of influence of mirror surface defects on distal light field are proposed. Influence of different mirror defects on laser beam quality is studied by simulation. Variation rules of parameters δ and correlation τ are obtained, ie, intrinsic linear relationship between parameters δ and correlation τ. For any TEM₀₀ mode, TEM₀₁ mode and TEM₀₂ mode, this linear relationship is similar and universal. For example, when transmittance rate of mirror surface defects is 0, slope of linear curve that δ varies with τ is equal to 1.8 approximately, despite of whichever mode laser beam is.