In order to investigate the focusing ability of half wave acoustic band lens，the geometric formula of the two-dimensional ( 2D) half wave acoustic lens is obtained by using the Fresnel diffraction principle. A 2D axisymmetric physical model of the half- wave acoustic lens is established，and the finite element method is used to simulate the model-Holmholtz equation. Based on the physical model，the influence of the frequency，the number of half band and the transmission coefficient of the acoustic lens on the focusing ability are investigated. The results show that the increase of the number of half-band increases the sound pressure level at the focal point and change the position of the focal point. The lower the frequency is，the smaller the focal length is. The larger the transmission coefficient is，the smaller the sound pressure level at the focal point is. Finally，the effectiveness of the physical model is verified by making a simple acoustic lens.