Abstract: Frost is one of the main agricultural meteorological disasters restricting the production of economic fruits. To study the frost index could provide effective reference for frost forecast, monitoring, early-warming, frost risk assessment as well as disaster prevention. The author focused on the critical temperature and time of duration of suffering frost in full blossom and young fruits of apricot and plum by simulated frost experiment using plant growth chamber. The results showed that: there was no significant difference in ‘Jintaiyang’ and ‘Limeixing’ concerning the index of frost, light frost would happen when temperature reached -2℃, and medium frost would happen when temperature reached -2℃--3℃, and when temperature was below -3℃ and lasted for 1 hour，the flower would suffer serious frost. ‘Yousa’ had the strongest cold resistance in 4 plum varieties, for which the light frost would happen when temperature lasted for 4 hours at -3℃, but for the other varieties, serious frost would happen when temperature reached -3℃, and ‘Longyuanqiuli’ had the worst cold resistance, its rate of frost injury would reach 50% when temperature lasted for 1 hour at -3℃. For young fruits of apricot, light frost would happen when temperature lasted for 1-2 hours at -2.5℃, medium frost would happen when temperature lasted for 3-4 hours at -2.5℃ and heavy frost would happen when temperature lasted for more than 1 hour at -3℃ or below, there were no significant differences between the two varieties. The cold resistance of plum was weaker than that of the apricot in young fruits. For ‘young’ fruits of Yousa of plum, light frost would happen when temperature lasted for 3-4 hours at -1℃ or lasted for 1 hour at -2.5℃ and heavy frost would happen when temperature lasted for more than 2 hours at -2℃ or -2.5℃ or temperature below -3℃. For young fruits of ‘Lanmi’ of plum, heavy frost would happen when temperature lasted for more than 3 hours at -2℃ or -2.5℃or temperature below -3℃.