Abstract: Based on rate equations,changing doping concentration and cladding size of the Er3+/Yb3+ co-doped double-cladding fiber laser,the effect of the doping concentration and cladding sizeon the performance of the Er3+/Yb3+ co-doped double-cladding fiber laser was studied.Analysis results show:when Er3+-concentration is stable,the output power will be increased efficiently with Yb3+-concentration increase;when Yb3+-concentration is stable,the output power will be increased too with Er3+-concentration increase,but the change is not obvious;the optimal fiber length of the laser becomes shorter when the inner cladding size becomes smaller.