Objective To investigate the mosquito species composition and activity rhythm in the lower reaches of Ertix River on the Sino?Kazakhstan border and their relationship with the habitat. Methods The species, density, seasonal rhythm, and circadian rhythm of mosquitoes in different habitats were investigated by human?baited net traps. Biting cycles were determined through biting indices. Results Eight species of four genera of mosquitoes were found in the lower reaches of Ertix River on the Sino?Kazakhstan border. Aedes vexans was the predominant species, with a proportion between 82.22% and 96.09% among all captured mosquitoes in different habitats. The seasonal activity of Ae. vexans started in mid?April, ended in early September, and reached the peak between mid?June and late July. The circadian rhythm of activity was in a double?peak pattern, with the morning peak appearing at 1-2 h after sunrise and the evening peak at 1 h after sunset; the evening peak was higher than the morning peak. The biting circadian rhythm was dawn type; biting activity started around sunset and ended after sunrise. The biting circadian rhythm was also double?peaked; the morning peak appeared at 1 h before sunrise, and the evening peak appeared at 1 h after sunset. The species diversity index ranged from 0.1912 to 0.7227, indicating that mosquito species were not abundant in the lower reaches of Ertix River on the Sino?Kazakhstan border. The habitat had significant impacts on the species distribution and number of mosquitoes. The numbers of Ae. vexans, Anopheles messeae, and Culex modestus in riparian forest were significantly higher than those in saline regions. Biting of Ae. vexans was the severest in riparian forest. However, seasonal activity, circadian activity, and biting cycle were similar for Ae. vexans in different habitats. Conclusion The habitat has a significant impact on the population size of Ae. vexans. In integrated prevention and control of Ae. vexans, special focus should be placed on microhabitats where mosquitoes propagate.