Disclosed are a wavelength conversion device, a light-emitting device and a projection system, comprising a wavelength conversion layer having a first surface and a second surface opposite each other. The first surface receives an excitation light. The wavelength conversion layer absorbs the excitation to produce a converted light and emits the converted light or the mixture of the converted light and the excitation light from the first surface and the second surface. A scattering reflective substrate is stacked with the wavelength conversion layer and includes a white porous ceramic or a white scattering material for scattering the incident light. The scattering reflective substrate includes a third surface facing the second surface and scatters at least a part of the incident light on the third surface and then emits all the light from the third surface to the second surface.