Gain and loss characteristics play important roles for the fiber. Davidenko′s method is adopted to analyze characteristics of optical fibers with complex refractive index. The step-index fibers with complex refractive index in the core or cladding are discussed. Numerical results are in good agreement with those obtained by exact analysis. For the fibers with complex refractive index in the core, the gain deviation of HE11 and LP01 modes is about 0.6%. For the fibers with complex refractive index in the cladding, the above deviation is about 2%. In practical work, to get more accurate results, full-vector complex characteristic equation must be considered, especially for fiber with cladding of gain or loss.