A new type of spiral anchor-rock anchor composite foundation (i.e., double-anchor composite foundation) is adopted to adapt to the geological conditions of “soil-up and rock-down” which are widely existed in mountainous areas of power transmission line projects. Using the circular curve function to simulate the sliding surface of the soil body around the anchor of the composite foundation subjected to uplift load, and accordingly established the formula for calculating the ultimate pullout capacity of the double-anchor composite foundation considering the evolution of soil pressure coefficients at different locations of the sliding surface; established a numerical model of the double-anchor composite foundation by using Abaqus and analyzed the damage modes, materials, geometrical parameters, and geological conditions of the project for the ultimate pullout capacity of the double-anchor composite foundation. The influence of material, geometric parameters and engineering geological conditions on the ultimate pullout capacity of double-anchored composite foundation is analyzed, and accordingly, the engineering recommendations for the application of double-anchored composite foundation in mountainous power transmission lines are given. The results show that the calculation formula of the ultimate pullout capacity of the double-anchor composite foundation proposed in this study has high accuracy; increasing the number of anchor disks, the diameter of helical anchors and the strength grade of concrete will significantly improve the ultimate pullout capacity of the double-anchor composite foundation, increasing the number of anchor disks from 2 to 6, the diameter of helical anchors from 0.24 m to 0.48 m, and the strength grade of concrete from C30 to C70 will result in an increase in the ultimate pullout capacity of composite foundations by 26.9%, 75.8%, and 62.5%, respectively; the double-anchor composite foundation can play a greater pullout capacity in the soils with small cohesive force and large angle of internal friction, Compared to a double-anchored composite foundation in a soil with an internal friction angle of 15°, the ultimate pullout capacity of a composite foundation in a soil with an internal friction angle of 35° is increased by 74.5%.