In order to establish a technique to control the growth of grafted watermelon seedlings and to cultivate robust grafted watermelon seedlings，this paper took‘ Zaojia 84-24’ watermelon and bottle gourd rootstock‘ Qianggen’ as material，studied the effects of 12 treatments by different concentrations of 4 plant growth retardants as PP333，B9，CCC，Yuhualing No.3 on the biological characters and chlorophyll contents of rootstock and grafted watermelon seedlings； and comprehensively evaluated the effects of spraying plant growth retardant on the quality of rootstock and grafted watermelon seedling by the method of membership function. The results indicated that there were significantly different effects on biological characters of rootstock and grafted watermelon seedlings by different treatments. Using method of membership function，we found seedlings treated with 30 mg·L^-1 and 50 mg·L^-1 PP333 had higher quality and higher chlorophyll contents in rootstock and grafted watermelon seedlings. This suggested that spraying PP333 at 30 mg·L^-1 to 50 mg·L^-1 could effectively improve the quality of both rootstock and grafted watermelon seedlings.