Although the internal field can effectively maintain the separation between photo-excited charge carriers, the wide bandgap restrains ferroelectric materials from visible light absorption. This study examined the effects of transition metal (TM) Mn, Fe, Co or Ni doping on the structure and bandgap of Bi _3.15 Nd _0.85 Ti ₃ O ₁₂ (BNdT) prepared by the molten salt synthesis method. No other non-bismuth layered structure phases were introduced. Mn, Co or Ni doping does not change the three-layered perovskite structure of BNdT while Fe doping increases the layer number from three to four. The doping of TM ions decreases the bandgap obviously. Among them, Mn-doped BNdT shows the largest bandgap reduction by ~1.6 eV. The narrowed bandgap was discussed to be attributed to the electronegativity of TM ions and the lattice distortion induced by doping together. The present work provides an available way to control the bandgap of complex oxide materials and provides a new tool for manipulating oxide optoelectronics.