In order to improve the extraction efficiency, photonic crystal was generated on the ITO current spreading layer of GaN-based blue LED. By using the finite difference time domain method, the relationship between the structural parameters of photonic crystal and extraction efficiency was simulated. The optimization enhencement results of 27.93% were obtained on the photonic crystal with cycle of 300nm, duty of 60% and etching depth of 200nm. Point source location varied in a photonic crystal lattice cycle was investigated in the finite difference time domain method simulation to get the correction extraction efficiency by fitting the simulations. Using the effective medium theory to get the effective refractive indexes, extraction efficiencies of both TE and TM mode polarization were calculated according to Fabry-Perot thin-film interference model. The maximum difference between the TE and TM light out-coupling was 1.442, and the light source with high polarization contrast was obtained. The proposed device can be applied in the LED back-light to improve the energy consumption of the liquid crystal display.