The invention discloses a construction method of a high-speed railway filling subgrade in a soft soil area, comprising the following steps: (1) cleaning and leveling the original ground; (2) constructing the rigid piles within the influence line of construction load: after the site is cleaned and leveled and the subgrade is qualified upon inspection, constructing the rigid piles within the influence line of construction load; (3) reinforcing the soil mass between the toe and the influence scope line of the load for a compound subgrade; (4) constructing the bearing plate on a pile board structure; (5) laying a gravel cushion and a high-strength geogrid; after the construction of compound subgrade treatment measures on the bearing plate on the pile board structure and both sides, laying a mattress layer; (6) after the compound subgrade is qualified upon inspection, constructing the light foam concrete within the filling range. The invention may ensure the construction quality of the high-speed railway filling subgrade in the soft soil area, guarantee the reliability of construction of subgrade filling materials, guarantee the stability of the subgrade and qualified post-constructionsettlement of the subgrade, and guarantee the beneficial effect of no defect generated by the subgrade.