The assemblage structure and abundance distribution of jellyfish were studied based on the samples of Beibu Gulf during 4 seasons in 2006-2007. The environmental factors that probably affected the number of species, abundance and assemblage structure were analyzed. The study showed that a total of 125 jellyfish species were identified, which belongs to 4 assembleges, they were Hydromedusae, Siphonophora, Scyphomedusae and Ctenophore. Hydromedusae was the dominant assemblege, accounted for 79.2% of the total species. The species number of jellyfish varied seasonally, there was 83 in summer, which was the highest among 4 seasons, followed by winter, autumn and spring, the species number were 56, 49 and 49. The 20 meters deep of water could be considered as the boundary according to the horizontal distribution of jellyfish species, the species number of the above was more than the under. There were three ecological groups of the jellyfish, the neritic warm-water group such as Lensia subtiloides, Diphyes chamissonis and Pleurobrachia globosa was the dominant group, accounted for 61.6% of total species, followed by the high-temperature oceanic group, occupied 37.6%, Liriope tetraphylla, Aglaura hemistoma and Solmundella bitentaculata were belonged to this group, and there was only one neritic low-temperature specie identified, that was Ocyropsis crystallina. The abundance of jellyfish varied seasonally, was highest in summer, followed by spring, winter and autumn. The abundance in the northwest of the studied area was higher, and the northern of Weizhou Island was lower, basiclly less than 10.0 ind/m³. There were 17 seasonal species which appeared the whole year, 7 of them were dominant species, Lensia subtiloides, Diphyes chamissonis, Pleurobrachia globosa and Liriope tetraphylla were the most important. Lensia subtiloides got the largest abundance in summer, made contributions to 40.4% of the sum. Diphyes chamissonis' abundance was large in spring and winter, occounted for 36.3% and 32.6% of the sum in each season. The abundance of Liriope tetraphylla in autumn occupied 36.7% of the sum, was far more than the other three seasons. Pleurobrachia globosa' abundance in 4 seasons changed not much compared to the 3 mentioned species, was largest in summer, occupied 12.2% of the sum. The assemblage of jellyfish in northern Beibu Gulf was analyzed based on the effect of 10 factors, they were deeper (D, m), surface temperature (T_s,℃), middle temperature (T_m, ℃), bottom temperature (T_b, ℃), surface salinity (S_s), middle salinity (S_m), bottom salinity (S_b), surface chlorophll a (Cha_s, mg/m³), middle chlorophll a (Cha_m, mg/m³) and bottom chlorophll a (Cha_b, mg/m³). Though the results showed details difference in 4 seasons, it could be divided into two groups, the low-salinity group (lower than 20 m) and high-salinity group (deeper than 20 m) formed northern Beibu Gulf jellyfish assemblage structure. The low-salinity group was mainly affected by the coastal current from Guangxi to Vietnam, this group had two types depended on temperature, they were the neritic low-temperature type and the neritic warm-water type. The high-salinity group was mainly affected by the seawater. The environmental factors varied during four seasons. Currents and water masses were the main factors on jellyfish assemblage and abundance distribution.