Abstract: The limitation of classical adaptive optics (AO) system and the basic principle of multi-conjugate adaptive optics (MCAO) a system are briefly introduced. The isoplanatic angle's expression of single and dual-conjugate is deduced on the base of the relative reference, and the selections of conjugation height are thoughtfully discussed. The isoplanatic angles for single and dual-conjugate and classical AO system are compared in the condition of level and upright-propagtion. The simulation results indicate that MCAO system has a notable effect on the gain of the isoplanatic angle. single-conjugate AO system is suitable for short-distance propagation applications, but for the far-distance remote sensing, MCAO system performs better than the former.