Tube continuous rolling (TCR) is a beneficial method for the production of titanium pipes due to its high efficiency and shorter process, with the mandrel playing a crucial role. In this study, the mechanism of the mandrel and its influence on the TCR process were investigated in detail through numerical simulations and experimental tests. The analysis included a detailed examination of metal slip, tension, stress-strain field, temperature field, metal flow, and microstructure evolution for various mandrel diameter coefficient. As the mandrel diameter coefficient increases, the grains at the groove vertex are elongated and recrystallized grains appear. At the same time, the quantity of twins at the groove taper initially increases and then decreases. Therefore, the deformation mechanism at the groove taper changes from twinning to slip.