A wavelength conversion device, a manufacturing method thereof, and a related illumination device. The wavelength conversion device comprises a fluorescent powder layer (110) that is successively stacked, a diffuse reflection layer (120), and a high-thermal-conductivity substrate (130). The diffuse reflection layer (120) comprises white scattered particles for scattering the incident light; the high-thermal-conductivity substrate (130) is one of an aluminum nitride substrate, a silicon nitride substrate, a silicon carbide substrate, a boron nitride substrate, and a beryllium oxide substrate. The wavelength conversion device has good reflectivity and thermal stability.