The invention discloses a construction method for paving roads on debris flow soft foundations. The construction method comprises the following steps: investigating characteristic parameters of debris flow; calculating bearing capacity of a debris flow body; selecting and placing a subgrade box, wherein the subgrade box is a rectangular box body with a closed steel plate structure; selecting paving equipment; selecting a conventional backacting shovel of 1.0-1.6m<3> as hoisting and paving construction equipment; excavating one subgrade box by utilizing an excavator bucket with the backacting shovel, and transporting to a prepared paving position; placing the subgrade box on a levelled subgrade, paving in a manner that the long side of the subgrade box is perpendicular to the axis of the road, and adjoining the long side of the paved subgrade box. The construction method has the beneficial effects that the equipment is ensured to fast and safely pass through a sludge region, and conditions for entering of the equipment is created; and conditions that the large excavation equipment safely enters to the river side on a soft debris flow deposit body are created so as to develop the river dredge operation.