Objective To compare the difference between the volume CT dose index (CTDIVOL) and the size-specific dose estimate (SSDEDW) based on the water equivalent diameter (DW) in the measurement of radiation dose, and analyze the correlations of CTDIVOL, SSDEDW and dose length production（DLP） with age and, chest circumference（CC） and establish a regression model for rapid calculation of CTDIVOL, SSDEDW and DLP. Methods The imaging data of 748 children of chest were retrospectively analyzed. According to age, they were divided into three groups: <5 years old, 5~≤10 years old and 10~≤15 years old. We manually measured the area of interest (AROI) and CT value (CTROI) on the representative central cross-sectional image, calculated the SSDEDW based on DW, and compared the differences between CTDIVOL and SSDEDW at different age groups using t-test. Pearson analysis was performed to test the correlations of CTDI VOL, SSDEDW and DLP with age and CC. Consequently, we established regression models based on age and CC to calculate CTDIVOL, SSDEDW and DLP, and analyzed the accuracy of the models. Results CTDIVOL of all children was (3.05±0.72) mGy, SSDEDW was (5.69±0.87) mGy, CTDIVOL was 46.40% lower than SSDEDW, and the difference was statistically significant (t=-241.95, P<0.001). Age-based regression models were developed as follows: CTDIVOL: y=1.78exp(0.06x) (x=Age, R2=0.64); SSDEDW: y=4.16exp(0.03x) (x=Age，R2=0.51); DLP: y=40.10exp(0.09x) (x=Age，R2=0.81). Regression models based on CC were also established: CTDIVOL: y=-0.76+0.06x (x=CC,R2=0.84); SSDEDW: y=0.06x+1.99 (x=CC，R2= 0.55); DLP: y=-84.26+2.96x (x=CC，R2=0.81). The errors of CTDIVOL calculated based on the variables including age and CC were (0.13±0.10) and (0.09±0.06), respectively, and there was no statistically significant difference between them (t=11.92, P<0.001); the errors of SSDEDW calculated based on age and CC were (0.10±0.06) and (0.09±0.07), respectively, and there was no significant difference between them (t=-17.41, P<0.001); the errors of DLP calculated based on age and CC were (0.13±0.09) and (0.13±0.10), respectively, and there was also no statistically significant difference between them (t=-7.07, P<0.001); the error of radiation dose index calculated based on CC is smaller than on age. Conclusion In measuring the radiation dose of chest CT in children, CTDIVOL could underestimated its radiation dose compared with SSDEDW indicating SSDEDW could be more accurate. Using age and CC as the substitute indications of DW to calculate the radiation dose index of children before CT scan can quickly calculate their radiation before examination, which plays an important role in better control of radiation dose