Objective To establish a graded early warning system for dengue fever and assess its effect of early warning and to reduce the risk of dengue fever outbreak in Zhongshan, China. Methods A graded early warning system for dengue fever was established based on relevant literature and the data of vector surveillance and human case surveillance in Zhongshan from 1990 to 2007. The system was used for graded early warning in Zhongshan from 2008 to 2011. The outbreak scales before and after the system was established were compared based on the data on dengue fever outbreaks in Zhongshan from 1990 to 2011 to assess the graded early warning system. Results The graded early warning system for dengue fever was established according to the following indices: mosquito and oviposition positive index (MOI), human cases, and month of year; the system divided the probability of dengue fever outbreak into 5 grades, i.e., 0: almost no probability; 1: little probability; 2: probability; 3: high probability; 4: outbreak. The system sent grade 2 early warning signals 5 times, grade 3 early warning signals twice, and grade 4 early warning signals once from 2008 to 2011. From 2008 to 2011, there was only one outbreak of dengue fever involving 3 (1.20%) of 250 persons, which lasted for 20 d; from 1990 to 2007, there were 7 outbreaks of dengue fever involving 14-195 (median, 37) (0.84%-14.88%, median 3.81%) of all individuals, which lasted for 16-64 d (median, 42 d). Conclusion After the graded early warning system for dengue fever was established, the outbreak scale of dengue fever decreased significantly during 2008-2011, as compared with those during 1990-2007. The graded early warning system plays an important role in reducing the risk of dengue fever outbreak in Zhongshan.