The invention discloses a rolled steel heating furnace afterheat utilization system based on combustion air graded heating. A third-grade air preheater, a high-pressure superheater, a high-pressure evaporator, a medium-pressure superheater, a high-pressure coal economizer, a second-grade air preheater, a medium-pressure coal economizer, a low-pressure evaporator, a first-grade air preheater and a low-pressure coal economizer are arranged in a smoke afterheat recovery flue in sequence in a smoke flowing direction; smoke discharged by a heating furnace hearth is fed in the smoke afterheat recovery flue; a low-pressure drum-deaerator, a medium-pressure drum and a high-pressure drum are combined to form a low-pressure steam water system, a medium-pressure steam water system and a high-pressure steam water system; steam is generated to drive a steam turbine to act; and after external cold air is heated to reach a set temperature by the first-grade air preheater, the second-grade air preheater and the third-grade air preheater in sequence, the external cold air is fed in a burner of a rolled steel heating furnace. The system is reasonably distributed according to the grade of smoke heat energy to realize gradient optimal utilization of the energy.