In a Yb3+ laser, the two-energy level structure is close to the quasi-three-level model, but different from that for the Nd and Tm lasers, so it is necessary to investigate the quasi-three-level modeling that will be applied to the Yb3+ laser. Based on the energy level structure, cavity gain and loss as well as population distribution, we present the modeling. Introducing an effective cavity length factor, the laser intensity is calculated and the threshold is obtained. Comparison with the experiments, indicates that the effective cavity length ratio changes the fractional population function and loss, which would influence the threshold and output in turn. Applied to investigate the laser property in the process of end pumping Yb3+:YVO4 laser, we get the threshold being 1.1 W, corresponding to the L=1 mm and T=1%; whereas the threshold is 3.9 W, corresponding to the L=2 mm and T=10%.