In this study, the cucumber cultivar ‘Zhongnong 18’ and pumpkin cultivar ‘Jingxinzhen No. 5’ were used as materials, and the intact pumpkin seedlings (P) and the seedling with one cotyledon and apical meristem being removed (/P) were used as control, two grafting combinations including cucumber/pumpkin (C/P) and pumpkin/pumpkin (P/P) were set up, using the onecotyledon splice grafting method. The changes of the morphological indexes and starch metabolism in the rootstock cotyledon of grated seedlings were detected, and the effects of removing rootstock cotyledon on the growth of grafted seedlings were analyzed. The major purpose of this study is to reveal the role of starch metabolism of rootstock cotyledons in the growth of cucumber grafted seedlings, and thereby provide a theoretical basis for improving the quality of cucumber grafted seedlings. The results revealed that: (1) after grafting, the fresh weight and area of rootstock cotyledon of C/P and P/P seedlings increased rapidly, and the net increase in fresh weight and area decreased in this order: /P > C/P > P/P > P. (2) After grafting, the starch content in the rootstock cotyledons of C/P and P/P seedlings decreased during the early healing stage ［0-3 days after grafting (dag)］. Then the starch accumulated rapidly until 13 dag, when starch was gradually decreased. The starch content and its starch branching enzyme (SBE, for starch synthesis) and βamylase (βAL, for starch hydrolysis) activities in C/P rootstock cotyledon were significantly higher than that in P/P seedlings. (3) Removal of rootstock cotyledons at 0-10 day could significantly inhibit the growth of scion and roots in C/P grafted seedlings. Also, the root vigor, as well as the soluble sugar content and CWIN and HXK gene expression levels were significantly repressed. Removing the rootstock cotyledon at 0 dag led to the strongest inhibition effect. These results suggested that in the C/P grafted seedlings, rootstock cotyledon can serve as a storage organ. During early growth of grafted seedlings, the photosynthetic products can be stored as starch, and then the stored starch is hydrolyzed into monosaccharides, which can be used for the rapid growth of scion and roots of grafted seedlings.