Objective To explore the effects of chest scanning parameters of 2 Philips computed tomography (CT) models on image quality under free breathing. Methods Eighty patients examined on Philips 256-multislice CT (40 cases using conventional scan mode and 40 cases using fast scan mode) and 80 patients examined on Philips 16-multislice CT (40 cases using conventional scan mode and 40 cases using fast scan mode) were prospectively enrolled. In the 256-and 16-multislice CT conventional scan groups, the conventional spiral mode was selected under the control of breathing after deep inhalation, and the pitch was 0.977 and 1, respectively. In the 256-and 16-multislice CT fast scan groups, the fast spiral mode was selected under free breathing, and the pitch was 1.473 and 1.472, respectively. The objective evaluation indexes of image quality were compared among the 4 groups. The image quality was evaluated and scored by 2 senior chest diagnostic physicians, and the consistency of subjective scores between the 2 physicians was analyzed. Results The objective evaluation results of image quality showed density resolution:256-multislice CT fast scan group > 16-multislice CT fast scan group > 16-multislice CT conventional scan group > 256-multislice CT conventional scan group; spatial resolution:16-multislice CT conventional scan group > 16-multislice CT fast scan group > 256-multislice CT fast scan group > 256-multislice CT conventional scan group; and background noise:16-multislice CT conventional scan group > 16-multislice CT fast scan group > 256-multislice CT conventional scan group > 256-multislice CT fast scan group. The subjective evaluation results of image quality showed that the consistency of image quality score between the 2 physicians was good (Kappa=0.91). Conclusion The 256-multislice CT fast scan can effectively reduce image respiratory artifacts under free breathing.