Abstract: ObjectiveThe strains with strong or very strong cold tolerance at heading and flowering stage was screened out from elite rice strains of advanced backcrossing introgression lines to lay a foundation for breeding rice varieties with cold tolerance at heading and flowering stage.MethodSeed-setting rate of 146 elite rice strains of advanced backcrossing introgression lines,which were continuously treated for five days under low temperature of 18℃ at heading and flowering stage,were detected in phytotron.The relative seed-setting rate was the identification index for cold tolerance evaluation.ResultAmong 146 elite strains of advanced backcrossing introgression lines at heading and flowering stage,there were three strains of very strong cold tolerance (grade 1) which accounted for 2.06% of the total materials,19 strains of strong cold tolerance (grade 3) which accounted for 13.01% of the total materials,89 strains of moderate cold tolerance (grade 5) which accounted for 60.96% of the total materials,35 strains of weak cold tolerance (grade 7) which accounted for 23.97% of the total materials.No strain of very weak cold tolerance(grade 9) was detected.The advanced backcrossing introgression lines with indica germplasm as donor in the test material contained more cold tolerance genes at heading and flowering stage.The plant strains with Gui 649 background were stronger in cold tolerance than the plant strains with Ce 253 background.The frequency appeared in plant strains with strong or very strong cold tolerance was high among the advanced backcrossing introgression lines with donors of Zhongnong 127,Zhongnong 128,Zhongnong 139,Zhongnong 143,Zhongnong 145 and Zhongnong 146.ConclusionTwenty-two strains with strong restorer genetic background and strong or very strong cold tolerance are screened out,which can be used as cold tolerant parents in rice cold tolerance breeding.