Porosity defect is the main problem for the double-sided laser beam welding of aluminum alloy T-joints. Based on its technology characteristics, the effects of the surface states, laser incident angles, welding speeds, heat inputs and laser distances on the porosity defects were studied in this work. The results indicate that the pure aluminium cladding layer is beneficial for reducing porosity defects. Using larger laser incident angle and welding speed are good for reducing porosity defects, the reason is that the fluid flow changes from convection to toward to keyhole, which can help pores escape from the weld pool. And, reducing heat input and increasing laser distance are bad for reducing porosity defects, the change of the keyhole shape and fluid flow are the essential reasons. It is necessary to ensure keyhole symmetry and pass-through, which can help the pores escape from the weld pool and reduce the number of the porosity defects.