Liquid metal embrittlement is a phenomenon in which the mechanical properties of a metallic material are significantly reduced after contact with liquid metal, and the microscopic mechanism of this phenomenon is still controversial. The grain boundary penetration mechanism has recently been widely recognized, but the theory is still deficient. To refine the theory of grain boundary penetration, in this paper, the liquid metal embrittlement mechanism of aluminum by gallium is obtained by in situ EBSD, combining it with the fracture morphology features and comparing the differences of the microscopic feature changes and the crack evolution process during the in situ tensile process of embrittled and untreated aluminum specimens. The results show that the fracture elongation of aluminum decreased by 60% after being embrittled by liquid gallium at 80 °C for 40 min, and the gallium atoms entering the aluminum interior decreased the grain boundary cohesion while promoting dislocation emission. Combining the experimental results and previous studies, we divide the fracture of aluminum after liquid metal embrittlement into three stages, namely, the grain boundary penetration stage, the local fracture stage, and the integral failure stage.