The absorption and emission properties of the concentrations and thickness of Yb3+-doped phosphate glasses have been investigated. The integrated cro ss section, absorption cross section, emission cross section, the spontaneous emission probability, and the effective line width of fluorescence of Yb3+- doped phosphate glasses were determined. The effect of the radiation trapping on spectroscopic properties was discussed. It was found that the effect of radiation trapping increases remarkably with the concentration and thickness of Yb3+-doped phosphate glasses. Radiation trapping causes significant lengthening of the measured fluorescence lifetime τf which is longer than the radia tive lifetime τrad. The error in measuring the fluorescence lifetime unde r low Yb3+ ion concentration (0.2mol%Yb2O3) is 3 0% and the error is 43% under high Yb3+ ion concentration (6mol%Yb2O3 ) 6mol%. Radiation trapping also significantly increases the effective line width Δλeff. I t will be increased by 14% and 30% larger under low and high Yb3+ ion concentration respectively.