Abstract: To explore the characterization and physiological basis of cold tolerance of rice planting in cold region,a rice cultivar planted in cold region with strong cold tolerance Longgeng 31 and a cold sensitive rice cultivar 9311 were used as test materials.Changes of the phenotypic traits and physiological and biochemical indexes under low temperature stress and brassinolides（BRs） combined low temperature treatment were studied.The Longgeng 31 had higher seed germination under low temperature and phenotypic resistance to low temperature.BRs treatment improved germination rate of Longgeng 31 and 9311 under cold stress at the bud stage.In terms of physiological indicators,the increase of proline（Pro） content of Longgeng 31 seedlings was greater under low tempearture treat ments,while the increase degree of malondialdehyde（MDA） and relative conductivity（REC） of 9311was higher under low temperature treatment.Both MDA and REC decreased under BRs combined low temperature treatment.In the aspect of endogenous hormone,under low temperature,the contents of IAA,ABA,and SA increased,while the gibberellin（GA） content decreased.IAA content decreased,whereas the GA content increased under BRs combined low temperature treatment,and the increase range of Longgeng 31 was greater.These results indicated that the physiological basis for cold tolerance of Longgeng 31 was better than that of 9311 while the treatment of the exogenous BRs displayed the change trend favoring improvement of cold tolerance.