[Objectives] Jellyfish have a complicated life history including the asexual generation of sessile life and the sexual generation of planktonic life. Scyphozoan species usually have an obvious sexual generation, which is commonly named as medusa. However, the asexual generation is the key to ensure the population survival. Almost all jellyfish blooms were related to the strobilation of a large number of polyps in field. Therefore, it is particularly important to understand the asexual reproduction of jellyfish and the development process of ephyrae after strobilation. Although Mastigias papus is a popular ornamental species in aquarium and a common species in warm waters, knowledge about the asexual reproduction of M. papus is still very limited. Therefore, we made a detailed observation of the asexual reproduction including the formation of planuloid, the attachment process and morphological changes from planuloid to polyp, as well as the process of strobilation, and the morphological characteristics from ephyra to juvenile medusa. [Methods] Morphological changes were observed and recorded using stereoscopic microscope or inverted microscope. [Results] The results showed that, as a widely distributed species, M. papus only showed one style of asexual reproduction, producing planuloid at the base of the calyx (Fig. 3). Planuloids metamorphosed into polyps 93 hours after release at 25 ℃ (Fig. 4). When the temperature raised to 27 ℃, constriction emerged on the calyx of the polyp and the polyp transformed into strobila, and then the ephyra larvae were formed at the top of the strobila. The ephyra would release 47 hours after strobilation with more and more frequently pulses. It took 21 days for ephyra to develop into young medusae. [Conclusion] Planuloid producing was the only asexual reproduction mode for M. papus, but the reproductive efficiency was relatively high: each polyp could produce one planuloid in an average of 2 days, and planuloid could metamorphosize to polyp within 4 days. Ephyra in turn could develop into juvenile medusa within 21 days. The rapid asexual reproduction rate makes M. papus a popular species in aquarium and also make jellyfish blooms in nature.