Dual-functional Lithium Lead Test Blanket Module (DFLL-TBM) was proposed by China for testing in the International Thermonuclear Experimental Reactor (ITER). When an in-TBM helium coolant tube breaks, high pressure helium will discharge into the Pb-Li breeding zones. The pressure shock in the breeding box will threaten the safety of TBM. The main objective of this work was to study the pressure wave propagation in the DFLL-TBM due to the In-box LOCA. CFD simulations and analysis on helium coolant tube break accident were performed in this paper. Six different break locations each with a small and a large break size (9 mm * 9 mm and 9 mm * 27 mm) were considered. Computational results indicate that intense pressure waves spread quickly from the break to the surrounding structure. The maximum pressure during the accident is even higher than the operating pressure of helium and its value is related to the location of the break. The longer distance between the break and the farthest inlet/outlet, the higher the peak pressure. Compared to the small size break accident, the pressure variation in TBM is more intense in large break accident.