No reports of measuring the focusing anisoplanatism of laser guide star were mentioned before because the guide star instantaneous wavefront measurement technique by using sodium is not mature. As directly verifying the confidence of artificial sodium guide star which takes the place of the natural star to become the reference of atmosphere adaptive optic correction, in this paper a new method is put forward that the focusing anisoplanatic error of sodium guide star can be measured via synchronous pulse detection if the atmosphere Greenwood time is constant. Results of correlation analysis show that it is feasible to establish an experimental system for the measurement. A high-precision wavelength and linewidth sodium guide star laser and a 1.3 m caliber telescope underly the experimental system for the technology research on the focus anisoplanatism. Experiment of detecting sodium guide star has been carried out. And synchronous detection about the returning light matrix of visible light of sodium and the natural star is achieved when both are in a coincident imaging path. In the experiment, the recovered single frame wavefront of sodium guide star is acquired, and experimental data analysis is carried out in several aspects: contrasting the coherence of the two types of wavefront images, comparing the Zernike coefficients, and simulating the imaging calibration. Results show that the imaging contrast in the light matrix of the plused sodium guide star is poor because of the limited experimental condition so that the precision of the recovered wavefront is restricted, but the two groups of wavefronts the sodium guide star wavefront and the natural star wavefront always keep nice comparability, which we can see from the recoverd synchronous wavefront images. And the Zernike coefficients of both are close in the first 36 orders, which we deduce from the Zernike anisoplanatic error curves. Simulation result shows that the acquired sodium wavefront data would contribute to calibrate the far-field images of natural star in astronomical observation activities. And the measured results, below 0.1 rad2, while wiping off the wavefront recovering error are close to the theoretical result. Result shows also that the low SNR of sub aperture imaging restricts the wavefront recovery precision to some extent, but the effective atmospheric wavefront distorted information has been obtained by sodium guide star experiment and it can help to correct the natural star images.