Abstract: In order to realize large mode area and single-mode fibers,a combining method of gain-guiding and indexantiguiding technique was adopted.Based on this method,the effect of the reflectivity of the resonator mirror on gain-guided and index-antiguided fiber lasers was analyzed in detail.Numerical simulation results show that the mode field distribution is varied with gain coefficient and negative index step,so the large mode area fiber with single mode operation can be realized by choosing optimal parameters,which is highly instructive for the design of large mode area fibers.