In order to study the influence of lobed mixer on infrared(IR) character of serpentine 2-D exhaust system(S2ES), three different lobed mixer S2ESs with lobe numbers of 12, 15 and 18 were designed. The influence of lobe number on IR suppression of S2ES was studied numerically, and the results were compared with a relevant annular mixer S2ES. The results show that, as the lobe number increases, thrust rises slightly with the maximum difference less than 0.1%; the reduction of lobe number changing from 18 to 12 enhances the mixture of core flow and bypass flow and reduces the plume temperature, making the plume radiation intensity attenuated by 18.1% at most; compared with annular mixer S2ES, the radiation intensity of lobed mixer S2ES reduces at least 20.9% and 26.9% in the direction vertical to the axis of nozzle exit on the side and lower detection planes respectively. In summary, it is appropriate to use mixer with fewer lobes in order to achieve better IR suppression.