Gleditsia sinensis seedlings were used as the research materials, and the development process of G. sinensis thorn was observed and recorded by scanning electron microscope, stereomicroscope and paraffin section method, to clarify the morphological and structural changes of G. sinensis thorn during development process, which provides theoretical basis and technical guidance for the development, genetic improvement and utilization and directional cultivation of G. sinensis. The results showed that: (1) the G. sinensis thorn is branch thorn, which originate from axillary meristem of G. sinensis seedling, and the development process is similar to that of the branches. (2) The structure of G. sinensis thorn is elliptical, the xylem has ringthickened vessels, and the growth rate of xylem is greater than that of phloem. (3) The developmental stages of G. sinensis thorn can be divided into 8 stages, there are no thorn stage (2 d after accelerating germination), thorn primordium stage (3 d), scale leaf basically completed stage (7 d), thorn differentiation stage (8 d), thorn basic structure formation stage (14 d), the period that thorn begins to become woody (30 d), the period when the thorn starts to brown (75 d), and the period when the thorn is completely brown (165 d).