The invention discloses a prefabricated combined waterproof roadbed suitable for a soft soil foundation. The prefabricated combined waterproof roadbed suitable for the soft soil foundation comprises afilling foundation bed and prefabricated block splicing side slopes arranged on two sides of the filling foundation bed. The filling foundation bed is filled with filling in a layered mode, waterproof geomembranes are arranged on the bottom surface and two sides of the filling foundation bed; the prefabricated block splicing side slopes are formed by splicing a plurality of prefabricated solidified waterproof packaging blocks, and the prefabricated block splicing side slopes are provided with prefabricated slopes which have the slope degree of 1 to (1-1) to 2.5. A large amount of muck, silt and construction waste are consumed by the filling foundation bed and the prefabricated block splicing side slopes, and the muck and the construction waste are recycled; and the waterproof geomembranesare arranged at the bottom edge and the side surfaces of the filling foundation bed, so that the prefabricated combined waterproof roadbed achieves the function of environmental protection, and improves the waterproof stability of the soft soil foundation.