On the basis of numerical simulation and experiments, the formation mechanism of solidification microstructures on the alumina ceramic surface by selective laser melting (SLM) is clarified. The research results show that as for the SLM of alumina ceramic, the forming condition of Bénard-Marangoni surface instability is satisfied. As laser energy decreases, the convection is close to its steady state. The preheating of substrate can change the Bénard-Marangoni convection state. Low laser power,fast scanning speed and low preheating temperature are all beneficial to the formation of a steady laminar flow on the liquid surface.