Abstract: [Objective] To determine whether presence of third sacral screw channels in adults and measure the its parameter by image digital measurement. [Methods] A total of 300 healthy volunteers aged 18~60 were enrolled into this study, with a half male and a half female. The geometric boundary of the third sacral safety zone was determined by three-dimensional reconstruction of pelvic CT scan data and the optimal screw path was generated. The width at the axial "narrow point" of the optimal screw path was measured on both sides, and the length of the trans-sacral trans-iliac (TSTI) screw channel and pedicle channel of the third sacrum were measured, additionally, the projection length and projection area of the TSTI channel of the third sacrum were measured. [Results] Of the 300 volunteers, 47 (15.7%) had a fixation path that could accommodate screw 6.0 mm in diameter for the transverse sacroiliac screw of the S3 vertebra.The male proved significantly greater than the female in terms of S3 TSTI channel width [left, (5.0±1.9) mm vs (4.3±1.6) mm, P<0.001; right, (5.0±1.9) mm vs (4.4±1.6) mm, P=0.002], TST1 channel length [(111.1±8.3) mm vs (98.6±7.8) mm, P<0.001], pedicle channel length [left, (36.6±3.2) mm vs (29.1±3.0) mm, P<0.001; right, (36.6± 3.1) mm vs (28.7±2.9) mm, P=0.002], the projection length [(18.4±2.0) mm vs (16.3±1.8) mm, P< 0.001], projection minor axis length [(16.6±1.6) mm vs (11.6±1.3) mm, P<0.001], projection area [(127.0±12.4) mm2 vs (96.4±9.4) mm2 , P< 0.001]. However, there was no significant difference between the left and right sides in terms of axial S3 TSTI channel width and pedicle channel length in the same gender (P>0.05). [Conclusion] In this study, 15% of adults are found a radiological safe third sacral screw channel, which could be used for fixation with transverse sacroiliac screw.