Abstract: In the explosive welding of tube sheet, the collision velocity between circular tube and tube sheet is an important factor affecting the quality of explosive welding. In order to calculate the collision velocity between circular tube and tube sheet during explosive welding, based on the theory of explosive welding and the theory of tube expansion, the corresponding relationship between the expansion radius of circular tube and the expansion speed is derived, and then the corresponding relationship between the gap between circular tube and tube sheet and the collision velocity can be obtained. In this paper, the calculation formula of radius and velocity after expansion of circular tube is obtained by theoretical analysis and simulation of expansion process of circular tube, which is verified by numerical simulation of explosive welding of tube sheet and explosive welding test of tube sheet. The results show that the round tube and the tube-sheet are closely combined, and the bonding interface is an ideal wavelet combination. There are no obvious defects such as holes and cracks in the bonding interface, and the bonding quality is good. Therefore, the gap between the circular tube and the tube sheet selected according to the formula is reasonable. This formula is reliable and can be used to calculate the collision velocity corresponding to different gaps between the circular tube and the substrate during implosion. The ideal collision velocity is obtained by determining the gap, so as to achieve good welding quality.