Objective To determine the species composition and seasonality of mosquitoes in different environments from 2012 to 2014, and to provide the scientific basis for the prevention and control of mosquitoes and mosquito-borne diseases. Methods The adult mosquito surveillance data by light trap method were collected from four districts, then the densities of different environments, months, and years was analyzed. Results From 2012 to 2014, 15 549 mosquitoes were captured and the overall average density was 1.79 mosquitoes per hour. Culex pipiens quinquefasciatus Say was the predominant species, accounted for 83.46%. The annual density peak was observed in July in 2012; duel annual peaks were observed in 2013 (April and September) and 2014 (March and October). In different environments, the density in livestock shed (2.40) was the highest in 2014 while that in farmyard (3.14) in 2013. Conclusion In general, ecological habitats observed in Haikou city, Hainan, China were highly conducive to mosquito proliferation. The predominant species is Cx. pipiens quinquefasciatus. And the livestock shed and the farmyard are the key sites where the main control efforts should be made. The key period of mosquito eradication is from March to October during a year.