The liquid crystal optical phase shifter applied in the mid-infrared band is prepared. The sapphire with high transmittance in the mid-infrared band is selected as the substrate, the ultra-thin indium tin oxide (ITO) film is used as the transparent electrode, and the mid-infrared liquid crystal with low absorption is selected as a medium. By applying voltage to the liquid crystal phase shifter, the liquid crystal molecules form a periodic phase distribution under the action of the electro-optic effect, thereby realizing the phase modulation of the mid-infrared laser. The 20 μm liquid crystal optical phase shifter prepared in this paper can achieve a phase modulation depth of 2.59 π under the irradiation of 39 μm mid-infrared laser. When the effective value of the switching voltage changes from 2.65 V to 337 V, the response time is 19 ms, which can achieve the fast response of the mid-infrared liquid crystal optical phase shifter under the condition of high cell thickness . Finally, the functional requirements of liquid crystal phase shifter in mid-infrared band can be realized.