The absolute band gap is improved by reducing the symmetry of the photonic crystal, two structures of the rectangle lattice 2D photonic crystal with rectangle dielectric rods is put forward. The modified plane-wave expansion method for calculating quickly the band structure is used to study the absolute band structure at high frequency field, parameters is optimized, when rectangle lattice 2D photonic crystal contains only one set of rectangle dielectric rods, an absolute band gap Δω=0.1265ω_e (where ω_e=2πc/a, a is the lattice constant ,c is light speed) is found at a high frequency field, which mid-frequency ω_ mid = 1.9256ω_e,Δω/ω_ mid =6.6%; when rectangle lattice 2D photonic crystal contains two set of rectangle dielectric rods, a larger absolute band gap Δω=0.203ω_e is also found at a high frequency field, which mid-frequency ω_ mid = 1.8597ω_e, Δω/ω_ mid =10.9%.