Three types of steels were designed on the basis of GX40CrNiSi25-12 austenitic heat resistant steel by adding different Mn contents (2wt.%, 6wt.%, and 12wt.%). Thermodynamic calculation, microstructure characterization and mechanical property tests were conducted to investigate the effect of Mn addition on the microstructure and mechanical properties of the austenitic heat resistant steel. Results show that the matrix structure in all the three types of steels at room temperature is completely austenite. Carbides NbC and M23C6 precipitate at grain boundaries of austenite matrix. With the increase of Mn content, the number of carbides increases and their distribution becomes more uniform. With the Mn content increases from 1.99% to 12.06%, the ultimate tensile strength, yield strength and elongation increase by 14.6%, 8.0% and 46.3%, respectively. The improvement of the mechanical properties of austenitic steels can be explained by utilizing classic theories of alloy strengthening, including solid solution strengthening, precipitation strengthening, and grain refinement. The increase in alloy strength can be attributed to solid solution strengthening and precipitation strengthening caused by the addition of Mn. The improvement of the plasticity of austenitic steels can be explained from two aspects: grain refinement and homogenization of precipitated phases.