Serial　robots　provide　an　ideal　alternative　for　friction　stir　welding　of　large-scale　spatial　curve　welding　seam　due　to　their　high　flexibility　and　large　workspace.　However,　the　low　stiffness　of　a　serial　robot　will　definitely　affect　its　welding　accuracy.　In　order　to　enhance　the　stiffness　of　the　serial　robot　in　completing　friction　stir　welding　tasks,　this　paper　proposes　a　method　of　constructing　the　hybrid　stiffness　index.　This　method　uses　the　sigmoid　function　to　process　the　dexterity　or　joint　limit　index　and　applies　it　to　the　stiffness　index　as　a　weight　coefficient.　Then　the　soft　stiffness　index　with　dexterity　constraint　or　joint　limit　constraint　is　constructed　respectively,　and　these　two　hybrid　stiffness　indices　have　the　same　dimension　as　the　stiffness　index.　Subsequently,　the　dimension　synthesis　of　the　ZK-500　serial　robot　is　completed　by　maximizing　the　global　soft　stiffness　index　with　dexterity　constraint.　Finally,　based　on　the　soft　stiffness　index　with　joint　limit　constraint,　a　joint　trajectory　planning　algorithm　for　the　ZK-500　robot　and　positioner　system　is　proposed.　The　result　of　simulations　shows　that　the　soft　stiffness　index　with　a　joint　limit　constraint　can　not　only　ensure　the　stiffness　performance　but　also　improve　the　smoothness　of　the　joint　trajectory　in　the　robot　trajectory　planning　task.　©　2020　The　Society　of　Manufacturing　Engineers