【Purposes】 Owing to the serious segregation of Cr and Mo in super austenitic stainless steel, a large number of Mo- and Cr-rich second phases precipitate during the hot rolling process, which leads to rolling cracking and delamination. Boron microalloying is helpful for preventing the σ phase precipitation. 【Methods】 The matrices and precipitate structures of boron-free and boron-containing S31254 super austenitic stainless steels and the distribution of elements such as Cr and Mo were analyzed experimentally. Combined with the experimental results, two grain boundary structure models of γ-Fe ∑9(221) and γ-Fe ∑11(113) were taken as the research objects. The segregation tendency of Cr, Ni, Mn, Mo, Cu, and Si at grain boundaries was analyzed, and the influence of B on the segregation behavior of these elements at grain boundaries was discussed. 【Findings】 It was found that all of these alloying elements can be solid soluble in the grain boundary structure model of γ-Fe ∑9 and γ-Fe ∑11. Both Cr and Mo tend to segregate at the grain boundary of γ-Fe ∑9 and γ-Fe ∑11, and Mo tends to segregate most seriously at the grain boundary. When B is at the grain boundary, the segregation of Cr and Mo at the γ-Fe ∑11 boundary is inhibited, but the segregation of Mo at γ-Fe ∑9 boundary is weakly inhibited. 【Conclusions】 The essential reason why the precipitated phase can be inhibited by the addition of super austenitic stainless steel to B is explained from the atomic level, which lays a foundation for the composition optimization of super austenitic stainless steel in the future.