An experimental investigation of ac pulsed argon arc welding of the AlMg alloy was carried out under the condition of the defects of the high temperature and thermal deformation. The Vickers microhardness profiles were measured by microhardness tester. The temperature of the calibration point was examined by FlirA20. And the microstructure of the welded joints was examined by means of Olympus optical. The experiment results show that the width of HAZ increases while the current increases and welding heat input increases. In the same current conditions, the pulsed welding heating heat accumulation become less, the microstructure is better and the microhardness become higher. Compared with argon arc welding, the energy on the AlMg alloy is lesser, and relatively concentrated, which make the temmereture of the whole HAZ rises slowly. It was effective to resist the less heat accumulation,the better microstructure and the higher microhardness of the clad layer and HAZ.