Objective To measure the length of lateral mass screw（LMS） in lower cervical spine based on the PACS and analyze the affecting factors to provide guidance for clinical screw placement. Methods A total of 100 patients diagnosed with traumatic cervical spine injury or degenerative cervical diseases who underwent cervical spine surgery in general hospital of northern theater command of PLA from December 2016 to December 2017 were selected

and harvested all the data of preoperative patients′ threedimensional cervical CT scan and sent them to PACS.The length of LMS from C3 to C6 was gauged separately

then patients were divided into different groups by height

weight

gender

age

pathogenesis and BMI and the data from different groups were compared and analyzed.Results The mean length of LMS from C3 to C6 was (14.58±1.20)mm

which was consistent with researches before.The mean length of LMS in male and female respectively were （14.81±1.17）mm and (14.00±0.28）mm

the difference was significant(P<0.05).The mean length of group high and group low divided by weight separately were （14.84±0.17）mm and（14.30±0.17）mm

〖JP〗the difference was significant(P<0.05).However

the mean length of LMS among those different groups divided by height

age and pathogenesis were not different completely

but for the BMI group，there was no difference（P>0.05）. Conclusion Gender and weight are affecting factors of length of LMS in subaxial cervical spine

and the length of segmental LMS is affected by height，age and pathogenesis，which can be instructive for the clinical screw placement of LMS.