In this paper, the 1030nm output power of Yb3+∶YAG laser end-pumped by 940nm laser diode(LD) is studied theoretically, where the model is developed based on the quasi-three-level rate equation. The pump absorption saturation （PAS）and re-absorption of Yb3+ ion are taken into account. The results show that, the significant re-absorption of Yb3+ ion in the lower laser level results in the high pump threshold. The loss resulted from re-absorption is affected by the medium length and the active ion concentration, and it implies that there is an optimal combination of the length and the concentration maximizing the output laser power. Pump absorption saturation makes the population of the lower pump level and the absorption coefficient decrease, and the output power is lower.