Objective To investigate the composition and dynamic change of mosquito species in Longnan, Gansu province, China and analyze the correlation between Japanese encephalitis (JE) onset and Culex tritaeniorhynchus, and to provide a scientific basis for JE prevention and control. Methods The distribution characteristics of JE cases and mosquito vector surveillance results were analyzed using descriptive epidemiology. The relationship between JE incidence and Cx. tritaeniorhynchus density was determined by linear correlation. Results A total of 215 cases of JE were reported in Longnan between 2017 and 2018, and the mean annual incidence rate was 4.11/100 000. JE onset occurred mostly between July and September. JE was observed mainly in adults (87.44% of individuals ≥ 20 years old) and in peasants from rural areas (91.16%). Mosquito vector surveillance results showed that Cx. pipiens pallens was the dominant female mosquito species that was entrapped, followed by Cx. tritaeniorhynchus. In addition, the dominant mosquito species varied with habitat. Culex pipiens pallens was the dominant species in residential areas, parks, hospitals, and peasant households (>75.00%), whereas Cx. tritaeniorhynchus was common in livestock sheds (61.51%) and peasant households (9.85%). In 2017 and 2017-2018, the monthly monitored density of Cx. tritaeniorhynchus was positively correlated with the number of JE cases one month later (r=0.983, P=0.001; r=0.937, P=0.006). Conclusion The distribution, density time, and seasonal variation of Cx. tritaeniorhynchus are closely related to JE onset. Understanding the correlation between the change in Cx. tritaeniorhynchus density and JE onset will provide insights into the early warning for JE prevention and control.