The beacon problem of adaptive optics (AO) is settled by the promotion of laser guide star (LGS), but the anisoplanatic error due to LGS will degrade the performance of AO system as well. Analytical expressions are given to evaluate the modal anisoplanatic error of atmospheric turbulence for LGS. Anisoplanatic error for Rayleigh LGS and Na LGS are also analyzed. Results show that the modal anisoplanatic error of 90 km Na LGS is obviously less than that of 15 km Rayleigh LGS without the angle deviation between LGS and object, but is greatly sensitive to the angle deviation than Rayleigh LGS. Good performance will be obtained with low order aberration mode correction in LGS AOS, and when angle deviation between beacon and object is larger than isoplanatic patch, low order modes should be corrected for effective correcting result.