Yb：Y3Al5O12(Yb:YAG) single crystals with Yb3+ doping level of 5.4at%, 16.3at%, 27.1at%, 53.6at%, and 100at% were grown by the Czochralski method. The effects of different Yb3+ doping level on the absorption spectra and fluorescence spectra were studied. The results showed that the real absorption coefficient of the absorption peaks increases linearly with the increasing of Yb3+ doping level in Yb:YAG crystals. Absorption band oscillator strengths of Yb：YAG crystals were calculated with Smakula formula and the effects of different Yb3+ doping level on the oscillator strengths were also studied. The concentration quenching of fluorescence was observed when the Yb3+ doping level reached 27.1at% in Yb:YAG crystals. The shape of fluorescence spectra of Yb:YAG could be significantly influenced when the Yb3+ doping level is higher than 53.6at%.