The characteristics of Z-scan curves for strongly nonlocal nonlinear medium with finite size,such as lead glass and nematic liquid crystals,are studied. By employing the transformation of the q-parameter,we obtain the normalized transmittance curves based on the thermal-lens model. It is found that the influence of the boundary condition on the transmittance can be neglected,and the peak-valley position and their difference depend on the incident power. Experiments are carried out in lead glass,and the characteristics of the curves are verified with different incident power,and the nonlocal parameters of lead glass are obtained.