Abstract: In order to determine the chilling injury indices of pitaya，6year trees of Chinese pitaya variety “Zihonglong” （Hylocereus polyrhizus Briton & Rose）at flowering and fruit stage were used as material. Simulating temperature in the low temperature process，4 different dynamic intensity and duration of low temperature were set from September to October in 2012.The minimum temperature of experiment was 4，2，0 and-2℃ respectively and continued 1d，3d and 7d，taking the basis experiment minimum temperature 20℃ as the control（CK）.In each experiment，relative electrical conductivity（REC），MDA content，SOD activity and soluble protein content of pitaya branch were determined，and morphological change was observed at the same time.The results showed that when the daily minimum temperature（Tmin）varied from 2 to 4℃ and continued more than 3d or Tmin varied from 0 to 2℃ and continued 1-3d， pitaya branch sufferred slight chilling injury（grade 1），when Tmin varied from 0 to 2℃ for more than 4d or Tmin varied from-2 to 0℃and continued 1-3d，pitaya branch sufferred moderate chilling injury（grade 2），when Tmin varied from-2 to 0℃ and continued more than 4d，pitaya branch sufferred severe chilling injury（grade 3），when Tmin was less than-2℃， pitaya branch sufferred extra severe chilling injury（grade 4）.These indices were used to verify the pitaya chilling injury degree in Luodian county and Chuzhenfeng county in Guizhou province in March 2012.After a low temperature process，the results were in accordance with the truth.The results could provide scientific basis for chilling injury early warning，prevention and planting division of pitaya.

Key words: Low temperature, Pitaya, Chilling injury, Relative electricity conductivity（REC）, Malondialdehyde（MDA）, Superoxide dismutase（SOD）, Soluble protein