[Objectives] Abandoning eggs after nesting was a rare biological phenomenon for breeding Red-crowned Cranes (Grus japonensis), but it does occur occasionally. So far, it was unknown whether the parents of Red-crowned Cranes abandon eggs due to internal or external disturbances. Egg was the reproductive unit and the starting point of growth and development of birds. The important role of eggshells should be to bear the weight of hatching parents, to exchange gas with the outside world, to prevent excessive loss of water in eggs, to protect embryonic development, and to form organs and tissues such as bones. By comparing the differences of ultrastructure and element content between abandoned and normal eggshells, we explored the reasons why G. japonensis abandon eggs, and hoped to provide a reference for the protection of Red-crowned Crane and the formulation of corresponding protection strategies. [Methods] From 2014 to 2020, 6 abandoned eggs and 6 normal hatching eggs of breeding G. japonensis were collected in Zhalong National Nature Reserve, Heilongjiang Province. We studied the eggshells ultrastructure, element content, the thickness and density. The ultrastructural differences were compared by scanning electron microscopy (Hitachi X-650, Japan), the element content was measured by inductively couple plasma spectrometry (Agilent 7500, USA), the thickness was measured by electronic micrometer (Mitutoyo 422-230/0﹣25 mm, Japan, ± 0.003 mm), the mass of eggshell samples was weighed by electronic balance (Sartorius BSA124S, Germany, ± 0.1 g)，and the density was measured by drainage method. The data were analyzed using SPSS (16.0 version). The thickness, density and element content of eggshells between abandoned and normal eggs were compared by t-test (Independent-samples). The thickness of surface layer of crystal, palisade layer, cone layer and shell membrane in the ultrastructure of the eggshell were compared between two groups respectively. All results were expressed as Mean ± SD, and P < 0.05 was taken to be statistically significant. [Results] The thickness and density of abandoned eggshell were 68.0% (P < 0.01) and 71.5% (P < 0.01) of normal eggshell respectively (Table 1). The thickness of the surface layer of crystal, palisade cone layer and eggshell membrane accounted for 5.5%, 86.9% and 7.6% in the normal eggshell, and the thickness of the surface layer of crystal, palisade cone layer and eggshell membrane accounted for 3.9%, 85.9% and 10.2% in the discarded eggshell respectively (Table 2). The ultrastructure of abandoned eggshell was different from normal eggshell, including uneven thickness of the surface crystal layer, no obvious boundary between the surface crystal layer and the palisade layer, cracks and strip cracks in the outer layer, tiny spherical hollow structures and cavity structures similar to karst cave in the cone layer of the fence, and fewer sheet-like protrusions on the surface of shell membrane fiber in the abandoned eggshell (Fig. 1). Among the 23 elements were measured, the contents of Ca, Na, P, Mg and K in abandoned eggshells were significantly lower than in normal eggshells (P < 0.05), and there were no significant differences in the contents of Fe, Zn, Cu, Sr, Ag, Ni, Ba, Cr, Mn, Bi, As, B, Ga, Mo, V, Co, Ti and Sc (P > 0.05) (Table 3). [Conclusion] From the analysis of eggshell structure and elements, the abnormal contents of eggshell elements, structural and functional defects may be one of the reasons for egg abandonment by their parents, which may be related to the interactions of habitat resources occupied by themselves, reproductive pressure with other birds, food availability and survival strategies.