In order to extend the life of the engines, and reduce maintenance costs, and improve the training and transporting efficiency, refering to civil aviation regulations, the designing principles of flexible take-off function were determined for an aircraft. And the desgining methods and application of flexible take-off scheme were introduced specifically, which can ensuring the security of takeoff at utmost by tow points, one of which is enabling pilots to have control over the actual thrust seting, and the other is the safety mergin of the takeoff performance. According to the specific flight mission in common use, takeoff performance was researched at the typical takeoff weight using flexible take-off technology. according to analyzed result，in the specific case, the allowable maximum flexible temperature is 20 ℃ above the basic temperature T0℃，at which the actual takeoff performance remained some safety mergin，and the security requirement was satisfied. Take off thrust was reduced to 85% of the maximum value. The life of the engines would increase about 13%. It is valuable for the desgining and application of flexible take-off technology on aircraft in our country.