For Al/Mg friction stir welded (FSW) joints, when the thickness of base metal is too large, thicker brittle and hard intermetallic compounds (IMCs) are easily formed along the interface, which makes joint formation extremely difficult. In this paper, the interface interlocking composite Zn interlayer is innovatively used to study the evolution of IMCs at the interface of thick plate Al/Mg FSW joint and the change rule of joint performance, which provides theoretical and practical basis for subsequent high-strength joining of Al/Mg FSW joints. The results show that a low melting point eutectic layer (Mg+ Al12Mg17) with an average thickness of 69.7μm is formed at the upper part of the magnesium side interface of the oblique butt joint, and IMCs with an average thickness of 42.7μm and 21.2μm are formed at the middle and lower part. The IMCs layer consists of Al12Mg17 and Al3Mg2。Compared with oblique butt joints, when interfacial interlocking compound Zn interlayer is used, Al-Mg-Zn phase (Al5Mg11Zn4) and Mg-Zn phase (MgZn2, Mg2Zn3) are generated locally at the interface, replacing the original Al-Mg IMCs with a minimum IMCs thickness of 3.9μm. The tensile strength of the joint is increased from 2.7MPa of oblique butt joint to 32.3MPa of interfacial interlocking composite Zn interlayer joint, which is related to interfacial interlocking effect and thickness reduction of IMCs.