A new composite system of azo polymer doped liquid crystal is reported. In this system, phase grating is formed after photoinduced trans-cis isomerization occurs on azo polymer and it separates with the liquid crystal. Through the establishment of a multi-exponential model the refractive index changes of phase grating in the He-Ne laser irradiation are studied. And through adding continuously variable voltage to polymer-doped liquid crystal sample, its response to the electric field is studied. At the same time, the evolution of monomer doped liquid crystal with light and electric fields is studied. The results have shown that the refractive index change in sample of photoinduced polymer doped liquid crystal is saturated after 45 s and the relaxation time is 10 min.The saturation and relaxation time are respectively 0.17 s and 0.9 s when the sample is monomer-doped liquid crystal. The grating diffraction efficiency will suddenly drop down when the electric field reaches 1.0 V, 1.4 V and 2.6 V and 4.0 V, while that of monomer presents no significant changes.