The present invention provides a fiber-based light source device, which includes a plurality of first, second or third light-emitting devices for respectively emitting a plurality of first, second or third lights, a plurality of first, second or third directional couplers for respectively coupling the first, second or third lights in their order so as to generate a first, second or third coupled light, a fourth directional coupler for coupling the first coupled light and the second coupled light so as to generate a fourth coupled light, and a fifth directional coupler for coupling the third coupled light and the fourth coupled light so as to generate an output light source. With coupling light beams from light-emitting devices and switching of those light-emitting devices in a sequential manner, the energy loss could be minimized and good heat dissipation could be obtained.