The work aims to understand the law of microstructural evolution of FSPed NAB alloy and obtain the microstructures with best mechanical properties. Friction stir processing (FSP) tool was improved based on heat input theories of FSP, and FSPed NAB alloy with various microstructures were prepared by previous and improved FSP tool, respectively. Vickers microhardness of FSPed NAB alloy was tested by microhardness tester, and the microstructures were observed by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The micro-structures of FSPed NAB alloy were mainly Widmanstatten α, band-like α, stream-like α and equiaxed α microstructures. The peak temperature for forming the microstructures gradually decreased, and the equiaxed α microstructures prepared by improved tool had the best mechanical properties. The equiaxed α microstructure included more α and κ phases, and very few β′ phase. The heat input during FSP can be significantly reduced by decreasing the diameter of the shaft shoulder and increasing the length of the stirring needle. The equiaxed α microstructure prepared at low peak temperature has the best mechanical properties, and the main strengthening mechanisms are fine grain strengthening, second phase strengthening and work hardening.