To understand fish species composition, community pattern, dominant species and seasonal variation in the continental shelf of northwestern South China Sea, four otter trawl surveys were conducted in January and August 2006, and April and November 2007, representing winter, summer, spring and autumn, respectively. Sixteen stations were assigned in each otter trawl survey along the coastlines from eastern Leizhou Peninsula to southeastern Hainan Island. Fish collection and data analysis followed the National Marine Survey Standards (GB/T12763.6-2007). Totally 262 fish species, from 102 families and 20 orders, were collected in the four seasonal surveys. The results revealed that the highest species diversity was in winter with 166 species recorded and the lowest was in spring with 135 species recorded; 161 and 155 species were recorded in summer and autumn, respectively. The Order Perciformes was the most important fish group in the continental shelf of northwestern South China Sea; the highest species diversity recorded was the Perciformes in all four seasons, and 40 out of 72 species recorded year-round belonged to the Perciformes. Species found in all four seasons contributed to 80.34% of total fish biomass. Ninety-one species found only in a single season contributed to 1.62% of total fish biomass. The spatial pattern of fish communities was analyzed using the hierarchical clustering and non-metric multi-dimensional scaling (NMDS), five fish groups were indentified. The Group Ⅰ, recorded in all four seasons, was located in the coastal waters of western Guangdong Province and consisted of 3 or 4 stations. The scale and the distribution area of the Group Ⅰ were stable among seasons. The Group Ⅱ, recorded in spring, summer and autumn, was located in the coastal waters of southeastern Hainan Island and consisted of 6-9 stations. The scale and the distribution area of the Group Ⅱ had seasonal variation. The Group Ⅲ, recorded in spring, summer and autumn, was located in the deep waters of eastern Hainan Island and consisted of 4 stations. The scale and the distribution area of the Group Ⅲ were stable. The Group Ⅳ as a transitional community, recorded in summer and autumn only, was located in the deep waters of eastern Hainan Island and consisted of 1-3 stations. The scale and the distribution area of the Group Ⅳ were small and changed largely with season. The Group Ⅴ as a mixed community, recorded in winter only, was also located in the deep waters of eastern Hainan Island and consisted of 13 stations. The scale and the distribution area of the Group Ⅴ were the largest among the 5 groups. Based on the analysis of similarities (ANOSIM), the results showed the significant differences (R=0.847-0.939, P <0.001) in fish community pattern for all four seasons. Although the station combinations in each fish community had seasonal variation, the correlation coefficients on the community similarities between spring and summer, between summer and autumn and between autumn and winter were 0.780, 0.630 and 0.777, respectively. The variations of fish community pattern are closely related to ocean currents and depth. The brackish fishes distributed in the cold eddy area of eastern Leizhou Peninsula and contributed to the Group Ⅰ. The deep sea fishes along the South China Sea Warm Current in eastern Hainan Island contributed to the Group Ⅲ. In winter with the strongest South China Sea Warm Current recorded in the deep waters of eastern Hainan Island, the area was dominated by fishes from the mixed Group Ⅴ. Because of the upwelling in the coastal waters of southeastern Hainan Island the area was dominated by fishes from the Group Ⅱ. In summer and autumn, the coastal current was strong and mixed with the upwelling in the deep waters of eastern Hainan Island; therefore, the area was dominated by fishes from the transitional Group Ⅳ. This study provides scientific evidences for further sustainable fisheries development and conservation in the continental shelf of northwestern South China Sea.