The invention discloses a SNP molecular marker linked with wheat grain length QTL and its application, belonging to the field of crop molecular genetics and breeding; the SNP molecular marker is KASP-KL-sau1, and the polymorphism is C / T; the KASP-KL ‑sau1 co-localizes with the wheat grain length QTL QKL.sau‑1B on the long arm of the wheat 1B chromosome, and the genetic distance between it and the QTL QKL.sau‑1B is 4 cM, and the nucleotide sequence of 35bp before and after it is shown in SEQ ID NO: 31. The KASP-KL-sau1 disclosed in the present invention is extremely significantly correlated with the grain length QTL QKL.sau-1B, presents the characteristics of closely linked markers, has high accuracy for molecular marker-assisted selection, and can significantly improve the long-grain varieties of wheat. The selection and identification efficiency is high, and the success rate is high. While improving the efficiency of breeding work, it provides a basis for the study of wheat grain length genes.