Experimental system of high power Yb3+-doped double-clad fiber amplifier pumped by diode laser was set up. Frequency upconversion luminescence in this fiber amplifier has been studied theoretically and experimentally. Analysis shows that the green fluorescence at fiber side and ends is due to cooperative luminescence. The cooperative fluorescence spectrum from pump side and signal side of fiber has been measured and compared with theoretical spectrum. The influence of signal and amplified spontaneous emission on frequency upconversion fluorescence has been discussed. The relational expression between the fluorescence intensity and pump power was described theoretically based on the rate equation and the relational curve was obtained by experiment.