Urban lake plays an important role in reducing the urban heat island effect. With the lake in the martyr park of Hunan Province as study object, we probed into the influences of lake area and lake shape index (perimeter-to-area ratio) on the lake cooling effect through field measurement and CFD (Computational Fluid Dynamics) simulation. Results show that: 1) Lake area is the major factor that affects the cooling effect of lake. The smaller lake area, the weaker the cooling effect. In the control area (area for comparison with no lake), the mean temperature increases by 1.8 ℃ in general, and the green land in lake increases by 4.1 ℃ to the maximum. 2) Lake shape index has a weak influence on the cooling effect of lake which increases slightly with the increase of shape index. 3) Under the condition that lake area remains unchanged, an excessive shape index will greatly affect the integrity of the water body, thus weakening the cooling effect.