The exploration of the appropriate red phosphor with good luminescence properties is an important issue in the de-velopment of current white light-emitting diode (WLED) devices. Transition metal Mn-doped compounds are fascinating luminescent materials. Herein, we performed a systematic theoretical study of the microstructure and optical properties of K2TiF6:Mn4+ using the CALYPSO structure search method in combination with first-principles calculations. We uncov-ered a novel structure of K2TiF6:Mn4+ with space group P-3m1 symmetry, where the impurity Mn4+ ions are accurately located at the center of the MnF6 octahedra. Based on our developed complete energy matrix diagonalization (CEMD) method, we calculated transition lines for 2Eg → 4A2, 4A2 → 4T2, and 4A2 → 4T2 at 642 nm, 471 nm, and 352 nm, respectively, which are in good agreement with the available experimental data. More remarkably, we also found another transition (4A2 → 2T2) that lies at 380 nm, which should be a promising candidate for laser action.