Abstract: Objective: To evaluate the application of different noise index combined with Z-axis automatic tube-current modulation technology, guided by body mass index (BMI), in CT chest scanning of AIDS patients with PCP. Methods: A total 30 cases of AIDS patients with PCP underwent routine dose and low dose scanning protocol with Z-axis automatic tube current modulation. The noise index (NI) for standard protocol was set to 10, the noise index (NI) for low dose protocol was set to 20, the range of tube current was set to 30-500mA, the tube voltage was 120kV, the NI of control group was 10, and the NI of low-group was 20. The volume CT dose index (CTDIvol) and dose length product (DLP) during CT scanning were recorded and the effective dose (ED) was calculated. The noise value was measured at pulmonary tip level, aortic arch level and pulmonary base level under the longitudinal window. The quality and subjective noise of CT images were evaluated by the 5-point system. Results: The subjective scores of the two groups reached the standard of clinical diagnosis. When NI was 20, the noise was higher than that when NI was 10, and the difference between the two groups was statistically significant (P<0.05). When NI was 20, the mean CTDIvol, DLP and ED decreased by 63.20%, 63.48% and 63.49% respectively compared to the low-dose group, and the difference between two groups was statistically significant (P<0.05). Conclusion: Under the guidance of BMI, by using Z-axis automatic tube-current modulation technology, the radiation dose of AIDS patients combined with PCP can be significantly reduced by setting a reasonable NI, while the quality of CT image meets the requirements of clinical diagnosis.