The present invention discloses a preparation method of polyester cord fabric with low denier and high modulus, including steps of: spinning, reducing melt discharge volume for extruding yarn with denier of 500-1000 D; using high-speed motor and heat roller, spinning speed of 6000-6500 m/min to produce raw yarn; performing initial and repeat twisting of raw yarn by straight twisting machine; performing weaving of twisted yarn using Dornier air-jet loom to produce cord fabric, when weaving and reeding-in, using two into a reed way, adding creel roller bearing and creel single spindle tension control belt to improve uniformity; performing dipping of cord fabric using two-bath dipping, adjusting temperature to 250-260 degrees Celsius, tension to 0.3-2.9 daN/each yarn of key area, running speed of yarn to 55-70 m/min. The polyester cord fabric with low denier and high modulus can reduce rolling resistance and fuel consumption, and improve performance.