“Iron spot” refers to the black, cyan and brown spots with metallic tin light condensed from the drawing lines on the blue and white porcelain. As a typical identification feature of blue-and-white porcelain of the early Ming dynasty, it has long been considered related to the use of imported cobalt with high-Fe and low-Mn. Although there have been sporadic reports over the years, due to factors such as submicron crystal size, glaze in homogeneity, element doping, and crystal segregation, its morphology and structure have not been fully studied, resulting in the color mechanism of “iron spot” unclear, and the view that “iron spot” as an identification standard for imported cobalt pigment being questioned. Combined with previous research, we found that Raman spectroscopy and scanning electron microscope equipped with energy dispersive spectrometer have great advantages in analysing ancient ceramic microcrystal structures. In order to further explore the composition and structure characteristics of “iron spot”, the ultra-depth three-dimensional video microscope, scanning electron microscope with energy dispersive spectrometer, and Raman spectrometer was used to analyze the crystal microstructure of “iron spot” in five blue-and-white porcelain samples of Jingdezhen imperial kiln in the Yongle and Xuande period of Ming dynasty. The composition of the samples’ white glaze area, blue color area and “iron spot” area was tested by laser ablation inductively coupled plasma emission spectroscopy. The microscopic observation results show that the diversity of crystallographic morphology and distribution in the “iron spot” area of different samples is the main reason for many visual perceptions: the octahedron and its massive aggregate crystals precipitated on the glaze surface present a point-like flashing visual perception; the dendrites that are oriented in parallel and well-developed are prone to the phenomenon of “tin light”; the frosted visual perception is caused by the close arrangement of dendritic and snowflake-like crystals; The excessive development of anorthite forms the raised brown spot to form a network structure. In terms of microstructure, the dendrites in the Yongle period are mainly composed of Mg2+-doped CoFe2O4-Fe3O4 solid solution, while crystals in the Xuande period are mainly MnFe2O4-Mn3O4 solid solution doped with Mg2+ and Co2+, and associated with reticulated anorthite. The above results show that an “iron spot” can be formed on blue-and-white porcelain fired with imported or domestic cobalt pigment. Since the crystals formed are all cubic inverse spinel structures, they have a certain similarity in macroscopic morphology. To sum up, this study clarifies the microstructure and composition characteristics of “iron spot” on blue-and-white porcelain from Jingdezhen imperial kiln in the Yongle and Xuande period of the Ming Dynasty. It reveals the coloring mechanism of “iron spot”, which provides a certain scientific basis for the identification of blue-and-white porcelain in Jingdezhen imperial kilns, and also provide some reference for the application of Raman spectroscopy and scanning electron microscopy spectroscopy equipped with energy dispersive spectrometer in the analysis of complex microcrystalline structures of ancient ceramics.