Forward amplified spontaneous emission (ASE) in Er3+/Yb3+ co-doped double clad fiber amplifiers is studied by use of rate equations and transmission equations model, Stark energy level splitting theory containing 1060 nm wave band radiation, Er3+-Yb3+ absorption and emission cross sections, and Runge-Kutta 4th-order method. A multi-theoretical model is founded to analyze the ASE spectrum generally by changing the pumping power and signal power. When the signal is strong enough, it only can change the output power by changing pump power without changing the spectrum figure. When the signal is small enough, two local peaks appear at the 1535 nm and 1543 nm at the ASE spectrum. The theory is validated by experiment.