In order to narrow the margin between the predetermined symmetrical parabola transmission error curve (TEC) and that of tooth contact analysis (TCA), a dual-layer method was proposed for the TEC optimization of spiral bevel gears. The method consisted of inner and outer layer optimizations which were conducted in turn. In inner layer optimization, pinion machine-tool settings were obtained based on local synthesis and TCA was carried out which gave TEC and contact pattern. An agreement objective was set up. The difference between TCA TEC and that of the theoretical one was diminished in least square sense to ensure agreement by tuning the four higher order modified roll coefficients in the relative motion formula linking the pinion and its cradle. In outer layer optimization, a symmetrical optimization objective was set up. The number of meshing points on the left and right branches of TCA TEC has been regulated until equal to each other to ensure symmetry by adjusting the displacement of the mean contact point along tooth depth. The method brought up is validated for a pair of spiral bevel gears. The results show that the method corresponding to the symmetry and agreement is indeed effective for TEC optimization of spiral bevel gears.