Cr²⁺ doped ZnSe nanocrystals are an important type of mid-infrared materials. ZnSe-based semiconductor nanocrystals with core-shell structure have shown excellent optical, electrical and catalytic properties. The defects of nanocrystals are one of the important factors affecting their properties. For example, nanocrystals with twinned structure have higher strength and hardness. In order to improve the comprehensive performance of doped nanocrystals, Cr²⁺:ZnSe/ZnSe nanocrystals with core-shell structure were prepared by two-step hydrothermal processes, using soluble Zn salt as Zn source, newly prepared NaHSe solution as Se source, and Cr(AC)₂ as doping source, and chemically stable nanocrystals at room temperature were obtained by sintering at 400 and 800 ℃ under protection of argon or high vacuum, respectively. Their structural and morphological characterization results show that the nanocrystalline size is mainly concentrated in the range of 20-30 nm, the shell thickness is about 2.6 nm, and the nanocrystals have delamination defects, thus twins are developed. The analysis results show that the twin plane is (111), the angle between the adjacent two crystal planes is 70.02°± 0.5°. The twin density of the sample increases with the increase of the crystallinity, which indicates that release of lattice distortion energy leads to formation of delamination and twin. Meanwhile, the twin growth can be explained by dislocation induced nucleation-growth. XPS analysis shows that Cr elements with a valence of +2 exist in the nanocrystals, and the reflection spectroscopy test shows that nanocrystals have an absorption band around 1775 nm, indicating that the prepared nanocrystals with twins have a promising mid-infrared luminescent property.