The invention relates to the field of laser technology, in particular to a fiber laser. To avoid affecting the system stability of the fiber laser due to the high core temperature of the gain fiber inthe laser, the present invention provides a fiber laser including a forward combiner, a backward combiner, a gain fiber, and a pump laser. The forward combiner and the backward combiner are disposedat the two ends of the gain fiber, the pump laser is welded to the pump input fiber of the forward combiner and the backward combiner; the refractive index of the core of the gain fiber is greater than the refractive index of an inner cladding, a first core grating and a second core grating are disposed on the core; a first cladding grating and a second cladding grating are disposed on the inner cladding; and the operating wavelength of the first cladding grating and the second cladding are between the output optical wavelength of the pump laser and the operating wavelength of the first core grating and the second core grating. The overall temperature of the gain fiber of the fiber laser is controllable and the system is stable.