A vacuum-molded orthotic insole (10) and a preparation method therefor. The orthotic insole (10) is filled with multiple particles (12) in an airtight outer layer. The outer layer is provided with an air passage thereon and has a plane that matches the plane of a sole in structure. When the sole steps on the outer layer in a right posture, the particles can move in the outer layer along with the stereoscopic structure of the sole, and air inside the outer layer can be discharged out of the outer layer through the air passage because of the weight applied by the sole. And then, the air passage is vacuumed until the particles are shaped to press against each other and cannot continue to move, and the air passage is sealed airtight. In this way, the orthotic insole capable of helping the sole to be put in the right posture can be quickly and customized.