The influence of concentration of rare-earth ions Er3+ and Yb3+ on gain and noise characteristics of Er3+/Yb3+ co-doped double-clad fiber amplifier is analyzed based on rate equation and light propagation equation. The numerical simulation result shows that, the amplifier has better noise characteristic with lower Er3+ concentration and larger gain with lower Yb3+ concentration, and there is an optimal Er3+ concentration to get the largest gain.