Based on rate-equation,amplified spontaneous emission(ASE) behavior in the Er~(3+)/Yb~(3+) co-doped double-clad fiber amplifiers under small signal amplification and different pump configuration is studied completely.This result shows that the ASE behavior under small signal amplification is different from that under large signal amplification.When backward pumping configuration is employed,ASE+ output power is larger than that of ASE- output power,and the contribution of each wavelength to ASE+ output power is larger than that of the wavelength to ASE- as well.Whether forward or backward pumping configuration is employed,ASE output powers decrease gradually with the increasing of core NA.When the ratio of cladding to core area is less than a specific value,the output power of ASE+ is larger than that of ASE- for backward pumping.