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Pure niobium girth weld seam is one of the most common structures in the process of superconducting cavity fabrication. However, the circumferential molten pool behaviour in electron beam welding (EBW) has been seldom studied. In this paper, a three-dimensional model, which is combined with the VOF model, is established to study the dynamic behaviour of circumferential molten pool in electron beam welding of 2 mm niobium plate. The influences of recoil pressure, surface tension and gravity are taken into account during the simulation process. The temperature field and flow field of the circumferential molten pool are calculated, the typical positions in the molten pool are analyzed, and the characteristics of the circumferential molten pool are summarized. During partial penetration EBW, the simulation results show that there is no obvious difference in keyhole evolution between flat welding and girth welding due to the supporting effect of the unfused base metal on the molten pool. After entering the full penetration stage, the tail of the molten pool extends rapidly under the influence of the Marangoni effect and gravity. In addition, the numerical result is in good agreement with the experimental data, which verifies the rationality of the mathematical model.