In this paper, the ingots of TNM alloy(Ti-44Al-4Nb-1Mo-0.1B at.%) were obtained using a non-consumable electric arc melting furnace. As-cast microstructure of TNM alloy was investigated. Liquid metal cooling Bridgman-type apparatus was employed in the heat stabilization experiment of TNM alloy and the effect of thermal stabilization time on the mushy zone of TNM alloy during directional solidification has been investigated. The results show the as-cast microstructure of TNM alloy consists of a major of (α₂+γ) lamellar colonies, network-like B2 phase, small γ phase and boride located at the interface of colonies. With the thermal stabilization time increasing, the solid/liquid interface will come to a planar one and the field of temperature and solute in the melt in front of solid/liquid interface will become more homogenous in thermal stabilization test. However, overmuch thermal stabilization time will make the melt contaminated by the crucible. The study shows the thermal stabilization time of 30~60 min is sufficient to provide a planar starting interface for directional solidification. Thermal stabilization affects the distribution of Al in solid and liquid phases near the solid/liquid interface of TNM alloy, and further affects the distribution of borides.