The present investigation aims at the thermal conditions for the TA15 titanium alloy laser welded bottom-locking joint. Bottom-locking featured specimens under three welding speeds were manufactured to study the grain morphology and tensileproperties. Finite element computation based on temperature field simulation was conducted in order to shed more light on themicrostructure characteristics of the bottom-locking joint. The results indicate a significant influence of the welding speed onthe thermal cycle, microstructure evolution and tensile performance. As the welding speed increases, the width of weld zone(WZ) is decreased significantly. At the same time, the columnar crystal features a coarser shape and its growth cross regionat the bottom of WZ becomes smaller with the increasing welding speed. Besides, the size of martensite decreases with theincreasing of welding speed. With the support of both numerical and experimental results, complex microstructure evolutionmechanisms were revealed involving thermal cycle related to the welding speed. In addition, the laser welded bottom-lockingjoint of TA15 titanium alloy is even fractured at the base metal, which exhibits the comparable tensile properties. The EDSresults illustrate the inevitable burning losses of stabilization element during the welding process. All these mechanisms areassociated with the bottom-locking joint arrangement and the welding speed-induced thermal cycle.