At present, the assumed temperature thrust reduction takeoff has been widely used in the operation of domestic civil aircraft. In order to assist flight crew in understanding the principle of rapid correction under non-standard conditions and to recognize its rationality and safety, the "weight-assumed temperature" relationship of ARJ21 aircraft is taken as the analysis object, and three typical non-standard conditions such as anti-ice, runway state and air pressure state are respectively investigated. Carry out the ARJ21 aircraft assumed temperature change characteristics analysis, correction principle research and correction results of the rationality verification. It is revealed that the essence of the assumed temperature correction principle under non-standard conditions is to match the actual takeoff weight with the thrust corresponding to a "sub-high-temperature weight" point, and the difference between this thrust and the minimum thrust required by the actual takeoff weight is greater than zero. And demonstrating the variation law of V_R and V_2 in the high temperature and small weight area corresponding to different airfield elevations. The results show that it is reasonable and safe to use the modified column of takeoff analysis table constructed according to this method to modify the assumed temperature under non-standard conditions.