Objective A?novel?variable-diameter cortical threaded screw used?in a modified cortical bone?trajectory?(MCBT)?was?designed to verify?its mechanical properties using the MCBT technique. Methods According?to?MCBT?technology,?the?screw?pitch?was fixed?at 2 mm, the total length was 45 mm, the diameter of the thick rod was 5.5 mm,?the diameter?of?the?thin?rod was?4.0–4.5 mm,?and?the?length?of?variable-diameter position connecting the?thick?rod?and?the?thin?rod?was?2 mm. The parameters?were?set based on three?aspects: variable-diameter position,?thread?depth, and?thread?type. Three-factor?and?three-level?L9?tests were conducted and??screw?models?were established.?The?torsion and the bending?and pull-out force?of?the?designed screws?were?calculated?based on?the?finite?element?method,?the?results?were analyzed using range analysis,?and?then?the?screw?models?were?determined. The three-dimensional (3D)?model?of?L4?vertebral?body?in?osteoporosis?specimens?was?established?and?screws were?placed?according?to?the MCBT?technique.?The pull-out force of the novel variable-diameter cortical threaded screw was compared with that of a conventional non-variable-diameter cortical threaded screw. Results?Range?analysis showed that screw?No.6?(variable-diameter position:?24 mm?from?the?screw?head,?thread?depth:?0.7 mm,?45° symmetrical?thread)?was the?optimal?screw. The anti-pull-out force of the No.6?variable-diameter cortical threaded screw?was?13.1%?higher?than that?of?the 4.5 mm?conventional?non-variable-diameter?cortical?threaded screw,?and?no statistical?difference in anti-pull-out force was found between?the No.6 variable-diameter cortical threaded screw and the 5.5 mm?conventional?non-variable-diameter cortical threaded screw. Conclusions The variable-diameter position has the smallest influence?on pull-out?force?of?the?screw,?the?thread?type?has?the?largest?influence?on?pull-out?force, and?the?thread depth has?the largest influence?on?torsion?and?bending.?Compared with that of?the conventional non-variable-diameter cortical?threaded screw,?the?variable-diameter cortical?threaded screw had a?smaller?front?end,?which?prevented?splitting?at the entrance point?of the screw.?The?screw?has?a large?diameter at rear?end, thereby showing?improved?pull-out?performance. The?results?provide?a new theoretical?basis?for?the?clinical?application?of?MCBT?technology.