This paper proposes a systematic and integrated methodology for the tolerance synthesis of spiral bevel gearbox. In this methodology, for the complex structure of this gearbox, dimensional tolerances and geometric tolerances are modeled by the screw theory. Parallel chains produced by the fits of gear shaft and holes are solved by a new integrated method based on localization tolerancing with contact influence method and screw theory. Leverage effect of geometrical structures on assembly deviation is considered to enhance the precision based on unified Jacobian-Torsor model. At last, bottleneck tolerances are identified for tolerance optimization by contribution analysis and sensitivity analysis. By means of this approach, tolerance synthesis results of spiral bevel gearbox are more accurate and comprehensive for practical application.