To increase the output power of incoherent fiber laser beam combination needs to improve the diffraction efficiency of grating. The traditional equal interval method greatly wasted the high diffraction efficiency section, while the unequal interval method makes the high diffraction efficiency section occupy relatively more combining laser beam. On the condition that grating frequency f=200mm-1, grating thickness t=2mm, such method would provide a 14% higher diffraction efficiency compared with equal interval method.