The morphology, distribution and crystallographic orientation of TiB in situ formed in TiB/Ti-6Al-4V composite coating by laser was observed and analyzed by TEM, EBSD systematically, in order to explore the formation mechanism of tubular TiB. The results show that, TiB showed two morphologies: rod and needle. And the morphology of TiB crystals is controlled by growth time and element concentration. Rod-like TiB nucleated in the hypereutectic region, and needle-like TiB nucleated in the eutectic and hypoeutectic region. The growth of (010)_B27 and (001)_BF planes will no longer be stable, when the the product of diameter and the growth rate of TiB exceed the critical value. The growth rate in the edge region of TiB is faster than the center due to the concentration of B. TiB eventually forms a tubular structure because B element is consumed by the growth of edges that further inhibits the growth of the central region.With the increase of diameter of TiB, the tubular structure appears more frequently.