The inclusions on the bonded interface were analyzed by optical microscopy (OA) and scanning electron microscopy (SEM), and the formation mechanism of oxide inclusions was investigated and discussed. The results show that the inclusions are mainly made of oxides. The chemical composition of oxide inclusions can also be related to vacuum degree inside the clad metal slab, in which silicon dominated at the vacuum degree of 15Pa or higher and the aluminum did at the vacuum degree of 0.1Pa or lower. It can be considered that the oxide inclusions were formed due to the oxidation of aluminum, silicon, manganese elements diffusing from the inside to interfaces to be bonded of metals. The experimental results also indicate that oxidation also become more evident with the reduction in vacuum degree, and the proportion of oxide inclusions increases to 50% from 15% with the decrease of vacuum degree from 0.1Pa to near 15Pa.