The structures of Ti alloy melted by cold-Cu mold non-consumable-electrode arc furnace change from equiaxed structure in chill layer to columnar structure, dendritic structure and equiaxed structure along the radial direction in turn. Based on temperature epuation, the relation between temperature grads of liquid- solid interface and time, the relation between the ratio of temperature grads of liquid- solid interface to solidification velocity and time were established. Used these relations and liquid-solid interface stability dynamic, the structure change was explained. The hardness on different location were different, which can be explained by theory of plastic deformation combined with quantitative metallogenetic.