To reduce the threshold and to improve the contrast of the conventional Holographic Polymer Dispersed Liquid Crystal(HPDLC) grating, a new type electrically tunable grating based on a Twist Nematic (TN) cell was designed. Firstly, the substrates of conventional HPDLC grating were processed to get an uniform alignment of liquid crystal in the grating, which makes the liquid crystal field depend strongly on the polarization of incident light. Then, a TN polarization modulator was placed in front of the grating to adjust the incident light polarization to change the refractive index of the liquid crystal field. Consequently, by matching the refractive index of polymer with the alignment of liquid crystal, the electrical-optical properties of the grating could be controlled. The experimental results indicate that the threshold voltage of the new HPDLC grating is as low as 0.75-0.8 V and the contrast ratio reaches 245∶1, which is 6-7 times of that of the conventional HPDLC grating. The HPDLC grating can work in a stable state for a long time.