Abstract:Based on the observation data of the daily average temperature in Changsha from 1961 to 2020 and the development period of double cropping rice from 1996 to 2020, through the comparative analysis of the applicability of different statistical methods and different boundary temperatures, this paper optimizes the research method of safe growth period of double cropping indica rice and applies it to the long term study of safe growth period of double cropping rice in Changsha. The results show that the continuous statistical method is adopted for the boundary temperature statistics. The air temperature of 8℃ is used as the sowing boundary temperature of early rice, and 22℃ as the full panicle boundary temperature of late rice. The number of days from the first day of 3 consecutive days when the air temperature ≥8℃ to the last day of 3 consecutive days when the air temperature ≥22℃ is used as the safe growth period of double cropping rice. These indices are suitable for the study on the safe growth period of double cropping rice in Changsha or other double cropping indica rice areas covered with plastic film. In Changsha Area, 80% of the guarantee rate ≥8℃ is on 21 March, and the sowing date with boundary temperature ≥8℃ is significantly ahead of schedule; the date of 80% guarantee rate of safe full heading of late rice is 9 September, and the safe full heading date is delayed but not obvious. The safe growth rate of double cropping rice is 80%, the number of days exceeded 179 d, and the safe growth period is prolonged significantly.