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In this work, a novel low-cost and robust approach is developed to covert Ti-bearing blast furnace slag (Ti-slag) to the main feed stock of photoassisted NH3 selective catalytic reduction (Photo NH3-SCR) of NO with high performance. The CaTi1–x Mn x O3−δ catalyst was directly extracted from the Ti-bearing slag with MnO2 in situ modification and Na2CO3 reformation followed by dilute hydrochloric acid leaching and water washing. Various methods were employed to characterize the structure changes of the slag in the preparation processes of MnO2 in situ doping, Na2CO3 reforming, and leaching. Mn element can be incorporated into the perovskite CaTiO3 with 0.16 to 1.79 wt % depending on the amount of MnO2 added in Ti-slag at 1500 °C. It is interesting to note that the amount of Mn incorporated into the perovskite decreases remarkably with increasing the additional amount of MnO2 in the in situ doped modification, which reaches a maximum of 9.54 wt % for the reformed slag of 5 wt % MnO2 addition. Moreover, most of the Si has been transferred to the sodium aluminum oxide instead of diopside, which is easily removed in the following leaching stage. Further semiquant crystalline analysis shows that there is 99 wt % of perovsvkite CaTi1–x Mn x O3−δ with 1 wt % hexamagnesium manganese­(IV) oxide in the slag after leaching with 5 wt % MnO2 addition, and the catalyst shows more than 93% NO removal efficiency at 300 °C in photoassisted NH3-SCR performance. The present study proposes a new route for the high value-added recycling of silicate minerals and wastes via structure-reforming with in situ optimization of secondary resources.