When the top surface is non-slip solid and the ratio between height and radius of crucible equals 2, the finite-difference method is adopted to carry on three-dimensional numerical simulation on the thermocapillary convection in detached solidification under microgravity. The distributions of velocity and temperature in the melt are obtained as the width of gap in crucible is selected as 0.1, 0.075 and 0.05, respectively. The results show that when the Marangoni number is small, there is one steady toroidal roll cell near the lower free surface, and the flow of melt is steady and weak. With the increase of Marangoni number, the flow is expanded toward the inner part of melt gradually and the velocity of flow on the lower free surface increases. When the Marangoni number exceeds the critical value, the flow of melt becomes unstable.