Based on the plane wave expansion method, the band structure of a square lattice two dimensional (2D) photonic crystal (PC) with rectangular dielectric rods in the air background is studied. When the length and relative permittivity of the rectangular rods are set as 0.3633a and 11.56, respectively, a triply degenerate state is found at the center of the Brillion zone in the band structure. The triply degenerate state consists of two linear bands that generate Dirac cones and an additional flat band intersecting at the Dirac point. This 2D PC can be related to a zero index material with effective permittivity and permeability equal to zero simultaneously. The finite element method is used to simulate the transmission characteristics of electromagnetic waves in photonic crystals. The simulation results show that the phase of the electromagnetic waves at the Dirac point frequency does not change, and their transmission characteristics are the same as those in zero refractive index material. With a concave lens made of such kind of PC, a sub-wavelength focusing with 0.63 λ half-peak width of focus and a 4.12 λ focal length is realized.