The composition of materials in a micro-/nano-devices plays a key role in determining their mechanical, physical, and chemical properties. Especially, for devices with a compositional change on nanoscale which can often be achieved by point-by-point direct writing technology using a focused ion beam (FIB), electron beam (EB), or laser beam (LB), but so far, nanoscale composition analysis of a large-area micro/nano structures with a variation composition remains a big challenge in cost, simpleness, and flexibility. Here we present a feasible route to realize large-area composition analysis with nanoscale spatial resolution by using Raman spectroscopy. We experimentally verified the capability of this method by analyzing a complex Sn-SnO_x system of a microscale grayscale mask with nanoscale spatial resolution of composition. Further analyses using Auger electron spectroscopy, transmission electron microscopy, and atomic force microscopy indicated the effectiveness and practicality of our method. This work opens up a way to analyze the composition of a large-area complex system at a nanoscale spatial resolution, and the method can be extended to many other material systems.