Abstract: The variation trends of the main crops yields in recent five decades were simulated by linear,quadratic,cubic and exponential models simultaneously.The results show that the main crops yields over years evolve in curve S trend in a long run,and their present yields are around its sharpest point.It will become more and more difficult for man to increase the crop yield from now on because cropping benefit has begun to decline.The maximal potentials of the main crops yields will probably be two to three times of their present levels.The approaches how to enhance the crop yield potential can be generalized as hydraulic infrastructure construction,breeding techniques,cultivation measures,input optimization and environmental improvement.