Due to its low cost, environmental protection, and excellent machinability, graphite free-cutting steel has attracted extensive attention from metallurgical and materials scholars at home and abroad in recent years. In this work, the effects of two kinds of carbon content (0.20wt% and 0.52wt%) on the microstructure changes of 1.9wt%Si-1.1wt%Al high-silicon and high-aluminum steel during hot-rolling, quenching-tempering process and the precipitation behavior of graphite particles after tempering were studied by metallography microscope, scanning electron microscope and energy spectrum analyzer. The results showed that the microstructure of 0.20wt%C experimental steel after hot rolling→quenching→tempering was transformed into ferrite+pearlite→ferrite+"island" bainite→ferrite+graphite particle+a small amount of cementite. The microstructure of the experimental steel at 0.52wt% C after hot rolling→quenching→tempering changes to ferrite+pearlite→martensite→ferrite+graphite particles+a small amount of cementite. At the same tempering temperature, the quantity and size of graphite particles in 0.52wt% C experimental steel are larger and more precipitated in grain boundary nucleation mode than that in 0.20wt% C experimental steel. When the tempering temperature increased from 680℃ to 710℃, the density of graphite particles precipitated on the grain boundary of 0.20wt% C steel decreased from 2337/mm2 to 1710/mm2, and the average size increased from 1.50 μm to 2.27 μm. At 0.52wt% C, the density of precipitated graphite particles at the grain boundary of the experimental steel decreased from 5244/mm2 to 1938/mm2, and the average size increased from 2.36 μm to 3.45 μm. The increase of carbon content from 0.20wt% to 0.52wt% can effectively promote the nucleation and growth of graphite particles in the tempering process. Compared with the tempering at 710℃, the number of graphite particles in the tempering at 680℃ is more and the distribution is more uniform, which is more conducive to the improvement of the overall machinability of the experimental steel.