The mechanisms of pore formation in the remelting zone under different protective atmosphere conditions are investigated by the laser remelting experiments of HT250 gray cast irons. The study results show that the pores in the remelting zone are dissolved pores under the argon shielding atmosphere, and the pores are dissolved pores and reactive pores under the open atmosphere. With the same process parameters, there are more pores in the melting zone under the open atmosphere than those under the argon shielding atmosphere at a low scanning speed, however, at a high scanning speed, the pores are basically the same under these two protective atmosphere conditions. When the laser energy density is relatively low, the main reason of pore formation in the remelting zone is the dissolving out of gasses from the gray cast irons.