The large mode area in conventional fiber is achieved generally by design fiber structure, which leads to limited mode area about several hundreds square microns. Gain guiding plus index guiding is a new method to realize single mode fiber with large mode area. By analyzing the effect of gain coefficient on refractive index and the complex-valued normalized frequency, the relationship between index-difference, gain-coefficient and cutoff conditions of different modes is obtained. Finally taking a Nd doped phosphate fiber with a 100 μm diameter core for an example, where the refractive index of 10% (atomic fraction) Nd-doped core is 1.5689 and 1.5734 in the cladding, through simulation computation, the mode-diameter is lager than 90 μm at 1.064 μm. To achieve large-mode-area single-mode fiber, it is a good method of applying gain guiding and index guiding for conventional fiber.