Traditional laser guide star (LGS) is difficult to detect the variance of atmospheric tilt because of the limitation of laser transmitter and receiver light path. An important solution is to use auxiliary telescopes to detect the LGS from the side, in order to single out the information of atmospheric tilt from the observed LGS strip. It leads to the phenomenon of anisophanatism with the increase of the length of LGS strip. The Zernike coefficient correlations between two spherical waves coming from the LGS strip are developed. From the point of correlation, the effect of the altitude of LGS, atmospherical turbulence and the caliber of the receiving telescope on the anisoplanatism of the LGS strip is analyzed. Furthermore, based on the anisoplanatism limitation of the tip-tilt determination concept, the request of field of view (FOV) of auxiliary telescope to view the sodium LGS is presented.