Based on the seedlings of corn and sunflower, we used the MSX-2F artificial frost box system to simulate the natural frost cooling process, leaf temperature was obtained by the thermocouple probe, combined with the frostbite and death statistics, we determined the temperature of super cooling point and freezing point of corn and sunflower, and determined the frost critical temperature in Inner Mongolia Hetao irrigation district. The results of the study showed that: (1)the temperature of corn be frozen to death is ≤-5℃, the temperature of sunflower be frozen to death is ≤-6℃; (2)the super cooling point of corn concentrated in the range of -3.5—-5℃, and the super cooling point of sunflower concentrated in the range of -4.0—-5.5℃; (3) There was very significant positive correlation exited between the corn leaf age, height and organization super cooling ability; there was very significant negative correlation exited between the corn leaf age, height and the temperature of organization super cooling, and the significant negative correlation exited between the corn leaf age, height and the temperature of freezing point. The analyses showed that the cold resistant ability of sunflower was stronger than corn, and the bigger seedling, and the higher the height, the lower super cooling point and the freezing point were, which showed that the stronger cold resistant ability was.