A revised plane wave expansion method for quickly calculating the band structure of photonic crystals of pixel type is used to design a photonic crystal of square lattice with a large absolute band gap at high frequency. The optimal GaAs photonic crystal structure has a large absolute band gap of Δ ω=0.0995ω e (where ω e=2π c/a, a is the lattice constant, c is light velocity) with a mid gap frequency of 1.2625ω e.