【Objective】 To understand the developmental characteristics of Paeonia ostii ‘Feng Dan’ seed and to provide theoretical basis for their scientific cultivation and management, we studied the morphological changes during P. ostii ‘Feng Dan’ follicle as well as seed development and discussed the transformation of nutrients during P. ostii ‘Feng Dan’ seed development. 【Method】 The P. ostii ‘Feng Dan’ follicles and seeds were stratified at six stages and used as materials for biological observation. Additionally, indexes such as water, soluble sugar, soluble protein, crude starch, crude lipid content, total amylase, total protease, acetyl-CoA carboxylase (ACCase), and phosphatidate phosphatase (PPase) activities in seeds were studied. Changes in nutrients and their related enzyme activities during seed development were analyzed. 【Result】 P. ostii ‘Feng Dan’ seed development was divided into two stages: green ripe stage (45-85 days after flowering) and brown ripe stage (95-115 days after flowering). During this process, the water content of P. ostii ‘Feng Dan’ seed decreased significantly, and the seed coat changed from yellow to white. Finally, the coat was hard and black brown. The pericarp also changed from green to yellowish brown and finally showed as grayish brown with dehiscent pods. Results of nutrient and related enzyme activity determination showed that during P. ostii ‘Feng Dan’ seed development, the oil accumulation flourished mainly at 45-85 days after flowering, increasing slowly at the middle and later stages, and reached the highest value of 33.1% at maturity. In addition to the consumption trend of soluble sugar corresponding to the accumulation trend of soluble protein, the change in soluble sugar content also showed a significant positive correlation with the change in crude starch content, showing downward-rising decreasing trends, and all reached the lowest value at 85 days after flowering. During the process, the trend of total amylase activity was opposite to that of crude starch, and the trends of total protease and soluble protein were the same. When the content of crude starch decreased with an increase in total amylase activity, the content of soluble protein increased with a decrease in total protease activity. ACCase and PPase activities also increased with an increase in lipid accumulation rate. 【Conclusion】 The changing trend of nutrient contents and related biological characteristics indicated that it is more appropriate to harvest P. ostii ‘Feng Dan’ seed at approximately 115 days after flowering. Additionally, distinct management schemes must be adopted at different stages of the planting process. A sufficient supply of water and phosphorus fertilizer should be ensured at 45-65 days after flowering. At 65-85 days after flowering, shallow irrigation should be used, and an appropriate amount of nitrogen and potassium fertilizer should be applied. However, in the middle and later stages of seed development, the water supply should be further reduced and a small amount of topdressing phosphorus should be reapplied.