The invention provides a construction method for widening a railroad bed, which comprises the following steps: 1, determining a construction position, cleaning the railroad bed to form a pouring area, and pouring a concrete cushion layer on a base of the pouring area; step 2, establishing a mixing station and preparing cement paste through the mixing station; 3, forming foam through foam light soil production equipment, and conveying the cement paste to the foam light soil production equipment through a pipeline to be mixed and stirred with the foam to prepare a foam light soil; 4, conveyingthe foamed light soil to the pouring area for pouring construction to form a bed width matrix; 5, setting a waterproof drainage system on the bed width matrix, wherein the pouring area is divided intoa plurality of sub-pouring areas in the extending direction of the railway line; pouring construction is conducted on the sub-pouring areas, and pouring is conducted on the sub-pouring areas in a layered mode.