<p>The process window for thin plate Al/Mg friction stir welded joints is relatively mature and can achieve high-quality connections. However, thick plate Al/Mg friction stir welded joints are very difficult to obtain good formation due to the significant difference along the thickness direction for temperature distribution and material flow in the weld. Welding process parameters, as an important factor affecting joint temperature and material flow, are a crucial prerequisite for achieving good formation. Specifically, the influence of the selection for material position on the temperature and material flow of thick plate Al/Mg FSW joints is still unclear. In this paper, the effect of material position on the thermal cycle, metal flow, and microstructure is thoroughly studied based on the design of inclined butt joints. The results indicate that the joint without macroscopic defects can be obtained when the Al alloy is only placed on the advancing side. This is attributed to the sufficient flow of Al and Mg when the Al alloy is placed on the advancing side. When the Mg alloy is placed on the advancing side, the higher peak temperature and longer dwell time along the thickness direction are obtained at the joint interface. Further combining with electron back scatter diffraction (EBSD) results, when the Mg alloy is placed on the advancing side, the thickness of intermetallic compounds composed of Al₁₂Mg₁₇ and Al₃Mg₂ at the upper of the Mg side interface has exceeded 130.8&#xa0;μm, much higher than the 44.3&#xa0;μm when the Al alloy is placed on the advancing side. The thickness difference of intermetallic compounds caused by material position also occurs at the lower of Mg side interface.</p>