Equation of state and phase transformation under high pressure of two phases GaN have been calculated by means of plane wave pseudo-potential method with generalized gradient approximation. The electronic density of states, band structure and optical properties of change mechanism have been discussed near the point of phase transformation. the transition pressure of wurtzite and rock salt structure GaN is 43.9 Gpa and 46.0 Gpa by means of the equation of state and enthalpy equal principle respectively; The typical direct bandgap semiconductor of GaN became indirect bandgap semiconductor material on the structural phase transition process. Compared rock salt structure GaN with wurtzite, the main peak of the dielectric constant increases, the intrinsic absorption edge shift to high energy apparently. The optical properties of rock salt structure GaN is worse than wurtzite structure in the low energy region.