Optical brightening agent (OBA) has been successfully applied to a high-yield pulp (HYP) bleaching process (peroxide/OBA, or P/OBA technology) because of its effectiveness for improving the optical properties of treated pulp. In this study, P/OBA technology was optimized for the peroxide bleaching of spruce thermomechanical pulp (TMP). The results obtained were further analyzed using data processing software, and the bleaching chemical cost was estimated. The results showed that the brightness ceiling (the highest “technical brightness”) of Koyama spruce TMP from conventional peroxide bleaching was 75.5% ISO. In contrast, the brightness ceiling of the same mechanical pulp could be raised to 85.9% ISO using P/OBA technology. At a low-brightness target, OBA is not effective when added to the peroxide bleach liquor, and thus is not economical for this purpose; however, at a high-brightness target, the P/OBA technology is effective due to the remarkable brightening effect of OBA. When the brightness target was 80% ISO, an optimized economic cost of 280.2 RMB/ton can be obtained under the “relay point” conditions of 2.57% peroxide and 0.6% OBA (on o.d. pulp).