To identify the species of freshwater jellyfishes (Craspedacusta) occurred in Huangsongyu Reservoir and Huairou Reservoir of Beijing and to explore their water quality-indicating practicability, we analyzed the morphological and genetic features of the jellyfishes sampled from the two reservoirs on September 9 and 10, 2022, respectively. Combined with the data of physio-chemical factors and zooplankton biomass from routine aquatic ecosystem monitoring, we explored the conditions for the occurrence of jellyfishes. The results showed that the morphological characteristics of Craspedacusta samples from Huairou Reservoir and Huangsongyu Reservoir were very similar, with the mean umbrella diameters of (15.8±3.1) and (13.2±1.9) mm, (342±48) and (268±3.4) tentacles, and (140±27) and (134±2) statocysts, respectively. Four main tentacles were obviously longer than others, with trochiformis nematocyst warts arranged annularly on them. Meaningfully, their gonads were different in shape and color. Huairou specimen had large white cystiform gonads, and Huangsongyu specimen had smaller gonads in grey. These morphological characteristics confirmed the jellyfishes in Huairou Reservoir and Huangsongyu Reservoir were C. sowerbyi and C. kiatingi, respectively. ITS base sequence was amplified and sequenced by PCR and DNA sequencing, then finally compared with other Craspedacustas sequences in GenBank, which resulted in 100% genetic similarities between the Huairou samples and C. sowerbyi, and between the Huangsongyu samples and C. kiatingi and thus well verified the morphological identification results. In detecting the driving factors to the appearance of jellyfish medusa, seasonal variations of zooplankton biomass and six physiochemical factors were analyzed for checking the sudden changes just before and/or after the medusa occurrence in both reservoirs. Results showed that the pH, dissolved oxygen, total nitrogen and phosphorus concentrations, and biochemical oxygen demand kept relatively stable through the summer-autumn months without observable variation in both reservoirs, indicating a mesotrophic to mildly eutrophic state. In early September when jellyfish appeared, surface water temperature continued declining to lower than 25℃, and the biomass of zooplankton continued increasing to higher than 1.2 mg/L. Rich food might meet the energy needs of jellyfish for sexual reproduction, and temperature declining might have functioned as the driving factor. We concluded that Craspedacusta are highly tolerant to eutrophication and have a preference for mesotrophic waters, and that sudden changes of water temperature (sudden rise or fall) and abundance of palatable food are important factors that stimulate the sexual reproduction of jellyfishes. Accordingly, the driving effects of temperature shock stress and nutrient limitation were analyzed, with a view to providing a reference for the mechanism study of freshwater jellyfish occurrence in future.