Objective To investigate the change in population density of mosquitoes and the resistance to commonly used insecticides against larvae and adults of Culex pipiens pallens in Tangshan city, and to provide a scientific basis for mosquito control for 2016 International Horticultural Exposition. Methods The mosquito lamp method was used to determine the population density of mosquito adults for International Horticultural Exposition park and surrounding area during 2013 to 2016. The larval immersion method and the adult exposure tube method were used to determine the resistance of Cx. pipiens pallens to commonly used insecticides. The DPS software was used to calculate the median lethal concentration and its 95%CI, virulence regression equation and Chi-square values. The resistance level of different insecticides was compared with Wilcoxon H test by SPSS 17.0 software. Results The average density of mosquitoes was 4.641 per lamp hour in Tangshan city from 2013 to 2016, with the predominant species Cx. pipiens pallens. The highest average density was found in the barn among all habitats (10.556 per lamp hour). Culex pipiens pallens were the predominant species in all surveillance areas. The population density of mosquitoes showed a single-peak throughout the year, peaking in July, being 10.854 per lamp hour. Larval Cx. pipiens pallens were susceptible to DDT, cypermethrin, temephos, chlorpyrifos, tetramethrin, bendiocarb and pyriproxyfen, with the resistance ratio of 0.88, 1.94, 0.82, 0.50, 2.18, 0.53, and 0.02, respectively. All of tested larval mosquitoes were intermediately resistant to bifenthrin with the resistance ratio of 14.00 and highly resistant to ethofenprox with the resistance ratio of 57.00. Culex pipiens pallens adults showed a mortality of 25.00% when contacted deltamethrin for 1 h and mortality of 50.00% when contacted permethrin for 3 h, all tested adults were resistant to deltamethrin and permethrin. Culex pipiens pallens adults experienced 100% mortality when contacted propoxur for 2 h, which was considered susceptible. Conclusion The predominent species were Cx. pipiens pallens, density peaked in July, when comprehensive control measures should be taken. More considerations are warranted about how to choose carbamate, organophosphorus and organochlorine insecticides, prevent or slow down the occurrence of the resistance.