To investigate the original as-cast microstructure of 27Cr44Ni5W3Al+MA ethylene cracking furnace tube manufactured by centrifugal casting, the phase composition and microstructure of the as-cast furnace tube were analyzed using XRD, OM, SEM and TEM. The results show that the original as-cast microstructure of the furnace tube is mainly composed of austenite matrix (γ phase) and fishbone-like multi-phase carbides at grain boundaries; the inside of the multi-phase carbides is lamellar M₇C₃, while the edge is blocky M₂₃C₆. In addition, two shapes of Ni₃Al (γ′ phase) are observed, namely the granular phase distributed near the junction between M₂₃C₆ and matrix, and the blocky phase adjacent to M₂₃C₆. Both of M₂₃C₆ and granular γ′ have the cube-on-cube orientation relationship with the γ matrix ( , , ; , , . , , ; , , ). In addition, a large number of dislocations are observed in the as-cast microstructure, with sparse distribution away from M₂₃C₆ and dense distribution near M₂₃C₆. The precipitation mechanism of γ′ and M₂₃C₆ during centrifugal casting was also discussed. The W element reduces the lattice misfit between M₂₃C₆ and γ phase, thereby promoting the precipitation of M₂₃C₆. The chemical composition of M₂₃C₆ and γ′ phase exhibit complementarity, and the precipitation of M₂₃C₆ further facilitates the formation of γ′ phase.