Agropyron cristatum (2n = 4x = 28, PPPP) is an important wild relative of common wheat and carries many desirable yieldrelated traits. The wheat-A. cristatum 6P disomic addition line 4844-12 exhibited high grain number per spike (GNS), high spikelet number per spike (SNS) and high kernel number per spikelet (KNS). In this study, five A. cristatum 6P deletion lines, five wheat-A. cristatum 6P translocation lines and genetic populations of these lines were used to map the highGNS locus from A. cristatum chromosome 6P, which were genotyped via genomic in situ hybridization (GISH) and fluorescence in situ hybridization (FISH) in conjunction with 6P-specific markers. According to the evaluation of the agronomic traits in two growing seasons (2015–2016 and 2016–2017), we found that the deletion lines and the translocation lines carrying the long arm of A. cristatum chromosome 6P (6PL) exhibited high GNS, SNS and KNS, and the high-GNS locus was ultimately mapped to 6PL (0.27–0.51). To verify the localization results, we created a new translocation line, WAT650a (5BL•5BS-6PL), that carried 6PL (0.35–0.42); this line exhibited higher GNS and SNS than the recipient parent Fukuhokomugi (Fukuho) did. Collectively, the high-GNS locus of A. cristatum chromosome 6P can be an important locus for improving yield traits in common wheat; the translocation lines with high-GNS locus can serve as novel and valuable germplasm resources for wheat breeding.