Abstract: Based on the measured altitude distributional adta of the turbulence structure constant（C_n² ）,this article analyses the effect of turbulent outer scale（L₀）on atmospheric transverse coherenee length（r₀）and isplanatic angle （θ₀）. The calculation result shows that r₀ andθ₀ will obviously increase when limited L₀ is col1sidered. The r₀ and θ₀ are much larger for large telescope at 2.2μm. The limit of θ₀ is likely to be 0.5 Mrad and may be even larger.