Based on rate equations, the method of changing doping concentration and cladding size was used to get the results of the characteristics of the Er3+/Yb3+ co-doped double-cladding fiber laser for the investigation of the influence of the doping concentration and cladding size to the Er3+/Yb3+ co-doped double-cladding fiber laser. The conclusion shows: when Er3+-concentration is stable, the output power will be increased efficiently with Yb3+-concentration increase; on the contrary, when Yb3+-concentration is stable, the output power will be increased too with Er3+-concentration increase, but the change is not obvious; the optimal length of fiber becomes shorter when the inner cladding size becomes smaller.