The solidifier employed in the production of flowable filling material from sewage sludge (urban sludge) pretreated with steel slag (metallurgical solid waste) was systematically investigated. Through single-factor experiments on solidifier admixtures, mineral powder and desulfurized building gypsum were selected as raw materials of the solidifier, with the formulation further optimized through orthogonal experiments. A flowable filling material based on the above solid wastes was developed, providing a new approach for the resource utilization of low-value solid wastes such as urban sludge and steel slag.