All-inorganic halide perovskite (CsPb2Br5) nanocrystals (NCs) have received widespread attention owing to their unique photoelectric properties. This work reports a novel strategy to control the phase transition from CsPbBr3 to CsPb2Br5 and investigates the effects of different treatment times and treatment temperatures on perovskite NCs formation. By controlling the volume of tetraethoxysilane (TEOS) added, the formation of different phases of perovskite powder can be well controlled. In addition, a white light-emitting diode (WLED) device is designed by coupling the CsPbBr3/CsPbBr3-CsPb2Br5 NCs@TEOS nanocomposite and CaAlSi-N-3:Eu2+ commercial phosphor with a 460 nm InGaN blue chip, exhibiting a high luminous efficiency of 57.65 lm/W, color rendering index (CRI) of 91, and a low CCT of 5334 K. The CIE chromaticity coordinates are (0.3363, 0.3419). This work provides a new strategy for the synthesis of CsPbBr3/CsPbBr3-CsPb2Br5 NCs@TEOS nanocomposite, which can be applied to the field of WLEDs and display devices.