Abstract: Using 5083 Al alloy and AZ31B Mg alloy as the research objects, the effect of welding time on the intermetallic compounds and lap shear properties of refill friction stir spot welded joints were mainly studied. Results showed that the Mg alloy had insufficient material flow behavior when the welding time was 1 s, thus void appeared after welding. Void disappeared with increasing the welding time. Liquid eutectic phase formed inside the joint because the oxide layers on the Al and Mg substrate surfaces were broken by the stirring of the pin and the welding temperature was higher than the eutectic temperature. Intermetallic compounds formed after the solidification of the liquid eutectic phase. The lap shear failure load firstly increased and then decreased with increasing the welding time. The maximum failure load of 3.1 kN was obtained when the welding time of 2 s was used.