Agropyron cristatum is a wild relative of wheat that harbours elite genes for wheat improvement. The wheat-A. cristatum 7P disomic addition line II-5-1 exhibits high grain weight. Here, to dissect the genetic basis of grain weight contributed by A. cristatum chromosome 7P in wheat backgrounds, four segregated populations of the addition line were developed and evaluated in two wheat backgrounds. The results showed that A. cristatum chromosome 7P can stably and significantly increase the grain weight by approximately 2 grams, mainly by increasing grain length at different grain weight levels of the wheat background. The locus for increased grain weight from chromosome 7P shows dominant inheritance independent of the wheat background. Moreover, two deletion lines and 23 translocation lines were identified by cytological methods and molecular markers, and an enlarged chromosome 7P bin map was constructed with 158 STS markers and 40 bin intervals. With the genetic populations of these deletion and translocation lines, the genetic locus of increased grain weight was narrowed down to bin 7PS1-2. Two translocation lines (7PT-A18 and 7PT-B4) with smaller 7P chromosomal segments exhibited an increase in grain weight, grain length and grain width simultaneously. These translocation lines carrying the 7PS1-2 chromosomal fragment will be valuable genetic resources for wheat grain weight improvement. Collectively, this study uncovers the grain weight locus from chromosome 7P and provides novel pre-breeding lines with enhanced grain weight.