The forming mechanism, diffraction efficiency and polarization characteristic of phase gratings written in cross-linked azobenzene copolymer films under different holography modes were studied. Jones vector was utilized to express interference field, involving orthogonal linear polarization interference mode (SP) and parallel linear polarization interference mode (SS). Based on SP and SS modes, pure refractive index grating (PRIG) without surface topology and surface relief grating (SRG) with unnegligible surface relief were fabricated respectively. Periodic refractive index distribution of PRIG induced under SP mode was observed in polarization optical microscope (POM). Quantitative topographic measurement in near-field optical scanning microscope (SNOM) indicates that SRG has a surface relief at 85.23 nm, but PRIG less than 5 nm. When reconstructed light is set to horizontal polarization, diffraction efficiency of PRIG reaches 22.2%, while SRG reaches only 1.65%. The zero-and first-order diffraction beams of PRIG are horizontally and vertically polarized, with polarization degree at 0.9969 and 0.9963. Diffraction beams of SRG are both horizontally polarized.