Abstract: The morphological and cytological basis of fruit shape variation was clarified using melon chromosome 11 chromosome segment substitution lines （CSSLs）， the quantitative trait locus （QTL） region controlling melon fruit shape was further narrowed down， and a foundation was laid for identifying key regulatory genes of fruit shape. CSSLs with the highest background recovery rate were selected to observe fruit shape phenotypic differences. Cytological sectioning was performed to compare cell sizes among materials with different fruit shapes. A segregating population carrying the target fragment was constructed for phenotypic variation analysis， and linkage analysis was performed to further narrow the QTL interval. Sequence variation analysis of the candidate interval and transcriptome sequencing were used to screen for candidate genes controlling fruit shape.Two heterozygous substitution lines with 100% background recovery rate were identified. Their self-pollinated progeny segregated into three genotypes （A， B， H）， showing significant differences in fruit shape between elliptical and round fruits， with a segregation ratio fitting 3∶1. Cytological observations revealed that the flesh cell size of elliptical melon fruits was significantly larger than that of round melon fruits. Fine mapping localized the target fruit shape QTL to a 3.5 Mb interval on chromosome 11. Through sequence variation analysis and transcriptome analysis， 26 candidate genes were identified.It was determined that differences in cell size are the primary cause of phenotypic variation in CSSLs. The target QTL was finely mapped， providing a basis for further fine mapping and functional studies of key fruit shape genes.