The detection of chemical speciation with spatial resolution of 30 nm has been implemented at BL08U soft X-ray spectromicroscopy endstation of Shanghai Synchrotron Radiation Facility. For each specimen, we scan two absorption images separately at two energies E1 and E2 near the absorption edge of the element with E1>E2. After calculating the signal ratio of each pair of the corresponding pixels’ optical density in the two absorption images, we can obtain the map of the spatial distribution of that element. Compared with the conventional analysis method of K-edge subtraction (KES), our ratio analysis method is more sensitive and accurate in identifying the spatial distribution of the interested elements, and the definition of the contrast threshold determining whether a pixel contains the interested element, is simpler and clearer in physics. Through the analysis and calculation of the spatial distributions of the oxygen element on a pre-oxidized polyacrylonitrile fiber cross-section, we can verify the effectiveness of the ratio analysis method compared to the traditional KES method. This ratio method provides a new analysis tool for fast mapping the distribution of trace elements.