In order to understand the characteristics of flower bud differentiation and endogenous hormone content of the single and double flowers of the same plant of Paeonia rockii ‘Guangonghong’, and to lay a theoretical foundation for the cultivation of different flower types of Paeonia rockii, we used the Paeonia rockii ‘Guangonghong’ as the test material to investigate the flowering of ‘Guangonghong’. Optical microscopy (OM) and scanning electron microscopy (SEM) were used to study the structural characteristics of flower bud differentiation and petal development. Contents of endogenous hormones in flower buds of single and double flowers of the strain ‘Guangonghong’ at different differentiation stages. The result shows: (1) the singlepetal flower of ‘Guangonghong’ has a longer flowering period than the double petal flower. The epidermal cells of the singlepetal flower petal are smooth and flat without obvious wrinkles, while the epidermis of the doublepetal flower petal has obvious folds. The flower bud differentiation time of the double flower was significantly earlier than that of the single flower, and the volume of the petal primordia was also significantly increased, which may be related to the stamen petalization in the double flower. (2) Low concentration of GA₃ is beneficial to the flower bud differentiation of single and double flowers of ‘Guangonghong’; high concentration of ABA promotes the formation of double flower primordia; the increase of ZR content is conducive to the flower bud differentiation of Paeonia suffruticosa. (3) Higher ZR/IAA, ZR/GA₃, ABA/GA₃, and ABA/IAA are beneficial to the flower bud differentiation of doublepetaled flowers of ‘Guangonghong’; the increase of ABA/IAA and ZR/IAA ratios are beneficial to flower bud differentiation of singlepetaled flowers of ‘Guangonghong’; the decrease of (GA₃+IAA+ZR)/ABA and (IAA+ZR)/GA₃ ratios promoted the formation of petal primordia and stamen primordia, respectively.