Abstract: The mathematical forecast model of extremely low temperature distribution was built to foretell the possible occurrence of the lowest temperature in 5 a, 5-10 a and 10 a durations in the area where Eucalyptus dunnii was introduced and grown based on the data recorded in Jianyang City of north Fujian Province between 1971 and 2005. It was forecast that the possibly lowest temperature in 5 a duration was -7.47 ℃, in 5-10 a duration was -7.47-（-8.5 ℃）and in 10 a duration was -8.68 ℃ respectively. The semilethal low temperature for different E.dunnii superior trees was determined by electrolyte leakage rate. The decision making of fitplanting region for different E. dunnii superior trees was respectively determined with ‘Reaction to the Extreme Environment’, and it was put forward that the comparatively reasonable decision making should be made with 10 a forecast duration.