Objective To investigate the seasonal fluctuation in mosquito density in Liaoning province, China and to provide a scientific basis for developing mosquito control strategies. Methods Light traps were set at 100 monitoring points in 14 cities in Liaoning province to monitor mosquito densities in rural and urban areas. Results From 2010 to 2012, a total of 67 034 mosquitoes were captured, with a mean mosquito density of 8.12 mosquitoes/(lamp·h). Culex pipiens pallens and Aedes vexans were dominant species, accounting for 44.4% and 32.9% of all mosquitoes. The mosquito density was the highest in barns, followed by in peasant households, parks, hospitals and in residential areas. The mosquito density showed a marked seasonal fluctuation in Liaoning province, with peak mosquito activity in August and September and less mosquito activity in May, June, and October. Conclusion The basic information on mosquito species, their density and seasonal fluctuation has been acquired, laying the foundation for mosquito control and prevention of mosquito-borne diseases.