Benefit from their near-unity photoluminescence quantum yield (PL QY), narrow emission band, and widely tunable bandgap, metal halide perovskites have shown promising in light-emitting applications. Despite such promise, how to facile, environmentally-friendly, and large-scale prepare solid metal halide perovskite with high emission and stability remains a challenging. Herein, we demonstrate a convenient and environmentally-friendly method for the mass synthesis of solid CsPbBr3/Cs4PbBr6 composites using high-power ultrasonication. Adjusting key experimental parameters, bright emitting CsPbBr3/Cs4PbBr6 solids with a maximum PL QY of 71% were obtained within 30 min. XRD, SEM, TEM, Abs/PL, XPS, and lifetime characterizations provide solid evidence for forming CsPbBr3/Cs4PbBr6 composites. Taking advantage of these composites, the photostability, thermostability, and polar solvent stability of CsPbBr3/Cs4PbBr6 are much improved compared to CsPbBr3. We further demonstrated CsPbBr3/Cs4PbBr6 use in flexible/stretchable film and high-power WLEDs. After being subjected to bending, folding, and twisting, the film retains its bright emission and exhibits good resistance to mechanical deformation. Additionally, our WLEDs display a superior, durable high-power-driving capability, operating currents up to 300 mA and maintaining high luminous intensity for 50 hours. Such highly emissive and stable metal halide perovskites make them promising for solid-state lighting, lasing, and flexible/stretchable display device applications.