To account for the microstructure evolution corresponding to the changed scanning speed, the thermal-fluid dynamic model of the meltpool during the selective laser melting (SLM) process of Ti6Al4V was established by numerical method to study the thermal characteristics and the melt flow behavior. Results showed that increasing the laser scanning speed would result in a lower peak temperature but a higher heating and cooling rate on the specimen. Both the meltpool size and its duration were reduced with the increased laser scanning velocity. Typically, a waved solid/liquid interface was observed at the bottom of the rear part of the meltpool as the time elapsed, especially for a larger scanning velocity. The melt flow velocity had a magnitude of hundreds of millimeters per second and showed almost a linear decrease with the increased scanning speed. Except for the change in cooling rate, the variation in flow velocities of the liquid metal consequent to different laser scanning speeds may be another possible reason for the observed microstructure change. The final result suggests that the scanning velocity must be carefully tailored to obtain the optimized combination of process parameters for industrial application, allowing for its adverse influence on the microstructure morphology and thermal stress/deformation caused by higher values.