The electric field modulation of photoluminescence in ferroelectric-optical materials as a novel in situ, non-damaging and real-time controllable method has drawn much research focus. The broad bandwidth emission of 33 nm and a tuneable luminescence contrast of 28% were achieved in Nd-doped Pb(Mg1/3Nb2/3)O3-PbTiO3 (Nd:PMNT) tetragonal ferroelectric-optical crystals arising from spontaneous polarization. The study of Nd:PMNT ferroelectric crystals under a cyclic, triangular alternating current voltage wave showed that the change in the photoluminescence intensity subjected to an electric field is mainly related to the 180° domain distribution, rather than the 90° domain. The physical process of electric field tuning luminescence is to control the 180° domain distribution using the external electric field, which thereby effectively tunes the luminescence. This finding restricts the limitation of a specific phase change region, which greatly increases the range of materials used and has guiding significance for research in the electric field modulation of luminescent technology.