Objective To understand distribution and seasonality of Culex tritaeniorhynchus in Chaoyang district, Beijing. Methods According to the Surveillance Methods for Vector Density-Mosquito (GB/T 23797-2009), mosquitoes were collected by the CO₂ traps. To determine the spatial distribution, the two types of sampling sites including park (or green spaces) with waters and human populated area were chosen in different geographical locations, and mosquitoes were trapped from August to September. To find out the seasonality, the mosquitoes were trapped per ten days from May to October in the same place. Results The proportions of sampling sites with Cx. tritaeniorhynchus between 2^nd Ring Road and 3^rd Ring Road, 3^rd Ring Road and 4^th Ring Road, 4^th Ring Road and 5^th Ring Road, outside 5^th Ring Road were 1/2, 3/3, 4/8 and 3/6 respectively, and there were no statistically different in different area subdivided by ring road (P=0.570); the proportions of sampling sites with Cx. tritaeniorhynchus of parks (or green spaces) with waters and human populated areas were 76.92% (10/13) and 16.67% (1/6) respectively, and were statistically different (P=0.020); the Cx. tritaeniorhynchus densities between 2^nd Ring Road and 3^rd Ring Road, 3^rd Ring Road and 4^th Ring Road, 4^th Ring Road and 5^th Ring Road, outside 5^th Ring Road were 0.09, 0.16, 0.31 and 4.97 mosquitoes per trap-hour respectively; the mean (m) and variance (S²) of densities of Cx. tritaeniorhynchus were 1.56 and 24.03, S²/m=15.42>1, which suggested that Cx. tritaeniorhynchus clumped distributed; the first appearance of Cx. tritaeniorhynchus was in the last twenty days of July, then its constituent ratio showed a trend from rise to decline and the peak period of its density was in September. Conclusion Culex tritaeniorhynchus tends to spread from suburb to city with the improvement of water conversation, and clumped distributed in urban area of Chaoyang district. Vigilance is needed to the epidemic risk of Japanese encephalitis, and public health department should strengthen the prevention and control measures.