A light emitting device and a projection system adopting same. The light emitting device comprises a coherent source generating coherent light, a diffusing element, and a light guide part. The diffusing element includes a first surface and a second surface opposite to each other. The diffusing element disperses the coherent light to generate incoherent light. The light guide part is provided on the side of the first surface of the diffusing element and guides the coherent light from the coherent source onto the first surface to form a first light path, guides a part of incoherent light from the first surface through the first light path, guides the remaining incoherent light through a second path, and separates the first and second light paths. The luminous flux of the incoherent light emitting through the first light path is less than that of the incoherent light emitting through the second light path.