Phosphorus plays an important role in the solidification, segregation and as-cast microstructure of superalloys. In order to control the as-cast microstructure and homogenization of IN783 alloy, the effect of phosphorus on the as-cast microstructure and the solidification sequence of IN783 alloy was investigated and analyzed. The as-cast microstructure of IN783 alloy is constituted by γ matrix mainly located in dendrite core and segregation zone, together with β phase, blocky Laves phase and P-enriched phase mainly in interdendritic zone. Phosphorus hardly dissolves in β phase and blocky Laves phase dissolves in P-enriched phase. Thus, phosphorus inhibits the formation of blocky Laves phase, but promotes the formation of the P-enriched phase in the as-cast samples of IN783 alloy. As a result, the segregation of Nb in the γ matrix located in the segregation zone is decreased, consequently decreasing the area of the zone. The solidification sequence of IN783 alloy is L→ L+ γ→ L+ γ+ β phase→ L+ γ+ β phase+ Laves phase→γ+ β phase+ Laves phase + P-enriched phase.