Objective To evaluate the mosquito density in Beijing and analyze the trends of mosquito breeding in different environments and to provide a scientific basis for urban mosquito control. Methods The adult mosquito density was monitored using carbon dioxide mosquito lamp and mosquito-oviposition trap, while the larval mosquito density was monitored by spoon method for large bodies of water and pipette method for small volumes of water (in containers). Results From 2010 to 2012, a total of 37 603 adult mosquitoes were captured in Beijing, where Culex pipiens pallens was the dominant species, accounting for 95.26% of all mosquitoes, followed by Aedes albopictus, accounting for 4.62%; the seasonal fluctuation curve showed that the peak of adult mosquito density appeared from July to August; the adult mosquito density was the highest in the park green lands, followed by residential areas, hospitals, and tourist attractions. The seasonal fluctuation curve for mosquito larvae showed that the peak of larval mosquito density appeared from June to July for both large bodies of water and small volumes of water (in containers); the positive rate of mosquito larvae was the highest in residential areas, followed by construction sites, sewers, and large bodies of water. Conclusion The surveillance Results of mosquito density and the trends of its seasonal fluctuation can provide effective guidance for urban mosquito control; attention should also be paid to the breeding places and density changes of mosquito larvae, and the association of mosquito density with mosquito borne diseases should be analyzed, so as to provide a scientific basis for the prevention and control of mosquitoes and mosquito-borne diseases in urban areas.