The main limit for a laser- guide- star adaptive- optics- system is the focal anisoplanatism. Firstly, based on the filter function of the atmospheric turbulence sensing geometry,the variance of the residual wave- front distortion resulting from focal anisoplanatism was derived, then the effective diameter d0 of the imaging system was derived. d0 was calculated according to the atmospheric turbulence model in Kunming and Xinglong. The calculation shows that the correction of the piston and the tilt is important for the adaptive optics system.