To meet the demand of the 10% weight reduction goal for automotive steel, the microstructure and mechanical properties of Fe-18Mn-Al-C steel with different carbon and aluminum contents were investigated under different solid solution treatments, and the deformation mechanisms of the experimental steels were elucidated. Aided by thermodynamic calculation, transmission electron microscopy (TEM) and in situ scanning electron microscope (SEM) analysis, it was shown that for the 18Mn-1.5Al experimental steel with about 20 mJ/m² stacking fault energy (SFE), the twinning-induced plasticity (TWIP) effect always dominated in this steel after different solid solution treatments under tensile deformation. With the 7 wt% aluminum addition, the SFE of austenite was affected by temperature and the range of SFE was between 60 and 65 mJ/m². The existence of δ-ferrite obviously inhibited the TWIP effect. With the increase in the solution treatment temperature, δ-ferrite gradually transformed into the austenite, and the n-value remained low and stable in a large strain range, which were caused by the local hardening during the tensile deformation. Due to the difference in the deformability of the austenite and δ-ferrite structure as well as the inconsistent extension of the slip band, the micro-cracks were easily initiated in the 18Mn-7Al experimental steel; then, it exhibited lower plasticity.