Objective To explore the influence of iterative algorithm on image quality in ultralow-dose CT scan of lung.Methods The thoracic model was scanned using different parameter combinations.The tube voltage was chose with 80,100 kV in ultra-low dose group and the mAs was selected by 10,15,20,25 and 30 mAs.The control group selected the parameters of 120 kV,30 mAs.All the images were reconstructed with filtered back projection (FBP group) and iterative algorithm (IR group).The image noise and effective dose (E) were compared.Results When tube current and voltage were constant,the image noise of IR group was lower than that of FBP group,and the difference was statistically significant (t =1.102-8.070,P＜0.05).When the tube current was constant,the image noise of the 80 kV with FBP group was higher than that of 100 kV with FBP group,and the image noise of the 80 kV with IR group was lower than the 100 kV with FBP group(t =-8.639-7.841,P＜0.05).Compared with the conventional low-dose with FBP group,the image noise of each ultra-low-dose with FBP group was significantly increased,and the image noise of (80 kV,10 mAs),(80 kV,15 mAs),(80 kV,20 mAs) with IR group was significantly increased,and the image noise of (100 kV,15 mAs),(100 kV,20 mAs),(100 kV,25 mAs),(100 kV,30 mAs) with IR group was significantly reduced (t=-8.140-23.028,P＜0.05).There was no significant difference in image noise between the groups of (80 kV,25 mAs),(80 kV,30 mAs),(100 kV,10 mAs) with IR and the group of conventional low dose with FBP (P＞0.05).Compared with the conventional low dose group,the E of the groups of (80 kV,25 mAs),(80kV,30mAs),(100kV,10 mAs),(100kV,15mAs),(100kV,20mAs),(100kV,25 mAs),(100 kV,30 mAs) was decreased by 75.9％,71.0％,79.8％,70.4％,60.3％,50.2％,40.0％,respectively.Conclusions The image quality of the ultra-low dose protocol (100 kV,10mAs) with iterative algorithm is similar to that of the conventional low dose with FBP group,and the radiation dose could be significantly reduced.