In order to study the pore formation mechanism of die cast magnesium alloy during fusion welding, local re-melting of die cast magnesium alloy AZ91D of 6 mm thickness utilizing laser has been conducted. Microstructure and pore morphology of base metal have been observed adopting OM and SEM. The size of pores has been measured by particle size analysis software Nano measure 2.1. The formation mechanism of various kinds of pores was analyzed. The results indicate that the porosity increases with the increase of laser power. Small pores, resulted from hydrogen, have smooth inner wall and inverted-trumpet shape. Large pores have irregular shapes and hackly inner wall with obvious traces scoured by metal. These pores are developed by shrinkage blowhole of base metal. Mathematical model on relationship of pore in re-melted zone associated with that in the molten pool was established