Abstract: Objective  To understand the composition and seasonal fluctuation trend of fly density in Shanghai area for a basis for scientific prevention and control of flies. Methods  According to the vector surveillance protocol of Shanghai, we conducted an ecological monitoring of flies at various surveillance sites in Shanghai area from March to November during 2023-2024. The surveys were carried out in four types of habitats at each monitoring site, including farmers’markets, external environments of catering establishments, residential quarters, and large greenbelts. The flies were captured in the selected habitats by cage trapping, identified for the species and counted. Finally, the fly species composition and its density were analyzed and compared. Results  A total of 2 430 traps were deployed, and 3 892 flies were captured during 2023-2024. The annual mean density was 1.54 flies/cage. The fly density was 1.65 per trap in 2023 and 1.43 per trap in 2024, which showed no significant difference (t=1.282, P>0.05). Fourteen fly species were identified, with Muscina stabulans (15.08%, 587/3 892), Chrysomya megacephala (13.36%, 520/3 892), and Boettcherisca peregrina (13.00%, 506/3 892) being the top three by count. However, B. peregrina (0.23 flies/cage), C. megacephala (0.22 flies/cage), and M. stabulans (0.20 flies/cage) ranked highest by density. Species composition varied seasonally, except for M. stabulans and B. peregrina being active during the entire monitoring period. In spring, the dominant species was Aldrichina grahami (24.08%, 202/839), while C. megacephala (16.15%, 493/3,053) was more prevailing in summer and autumn. The fly density fluctuation in Shanghai area between 2023 and 2024 exhibited a unimodal pattern, peaking from June to August, with statistical differences among different months in each year (χ2=46.498, 37.311, all P<0.001). Among different habitats, large green belts (1 505 flies, 38.67%) and farmers’ markets (1 032 flies, 26.52%) accounted for the majority of captured flies. The farmers’ markets had the highest average fly density (2.03 flies/cage), followed by large green belts (1.51 flies/cage), external environments of catering establishments (1.45 flies/cage), and residential quarters (1.22 flies/cage). Conclusion  M. stabulans and B. peregrina are predominant fly species in Shanghai area. The fly density peaks in summer and agricultural trade fairs shall be the key sites for fly control. Our findings suggest that integrated fly control in the key sites should be persistently strengthened during active fly seasons.