Objective To explore the effect of different weight groups of sugar and vinegar bait to monitor fly density and provide scientific basis for control of flies. Methods During March-November 2015, the author selected 4 habitats of Jiading Park, residential area, restaurant and farmer's market were selected, trapping was conducted by fly cage, fly density was calculated and analyzed using Excel 2003 and SPSS 13.0 software to test the density of flies in different weight groups by means of rank and test. Results Five families of flies were captured, which belonged to 25 genera and 42 species. Twelve species of flies were identified by national vector surveillance program (Trial Implementation), accounting for 51.49% of the total catch. The predominant species consisted of Mascina stabulans, Boettcherisca peregrina, Lucilia sericata, Aldrichina grahami, Musca domestica, and L. illustris. The density of flies increased with the increase of bait weight, and the difference was statistically significant (H_C=9.188, P<0.05). Flies began to appear in March and disappeared in November. The seasonality showed peaks in June and October. The highest density of flies was observed after 1 d of sweet and sour bait. Conclusion The species, density, distribution and seasonal dynamics of flies were obtained by using sweet-sour bait to monitor flies.