In order to improve the cementing and filling effects of the upper part of shale oil horizontal wells, the settlement stability evaluation method was used to design the particle ratio scheme with equal particle size, which can simulate the actual temperature and pressure conditions underground. And a new hydration activator that is suitable for the normal and low temperature environment was developed with alkali lime and magnesium sulfate. In conclusion, a ternary low density filling cement slurry composed of fly ash, micro silicon, and cement was created by the 29Si NMR quantitative analysis, and it was carried out in 5 shale oil horizontal wells of Yanchang oilfield. The results show that the cement slurry is the most stable when the optimum dosage of fly ash and micro silicon is 40% and 35% (mass fraction), and the activator works best when the mass ratio of alkali lime to magnesium sulfate is 3∶1. All the above reduce the residual amount of cement by 29.59% and fly ash by 68.61%, and increase the 3 d and 7 d compressive strength of low density cement slurry at room temperature by 129% and 92%, respectively. In addition, the water loss, water precipitation and thickening time of the ternary composite system totally meet the needs of cementing and filling. The application of 5 wells shows that the qualified sections account for more than 90%, and the cement slurry has good application prospect and large-scale popularization potential.