A wavelength-converting element includes a substrate, a wavelength-converting layer and a diffuse reflection layer. The wavelength-converting layer is disposed above the substrate. The diffuse reflection layer is disposed between the substrate and the wavelength-converting layer. The diffuse reflection layer includes an inorganic binder and a plurality of diffuse reflection particles. The diffuse reflection particles are mixed with the inorganic binder. The inorganic binder includes an alcohol-soluble inorganic binder or a water-soluble inorganic binder. A projection apparatus using the wavelength-converting element and a manufacturing method of the wavelength-converting element are also provided.