The phase transition during equilibrium solidification and cooling process，non-equilibrium solidification segregation and the raw of phase transition of cast duplex stainless steel CD3MWCuN are investigated by FactSage 8.2 thermodynamic calculation software. The calculation results indicates that the phase transition path of equilibrium solidification and cooling are asfollows　Liquid→Liquid+δ-Ferrite→Liquid+δ-Ferrite+Austenite→Liquid+δ-Ferrite+Austenite+Nitrogen→δ-Ferrite+Austenite+Nitrogen→δ-Ferrite+Austenite→δ-Ferrite+Austenite+Cr

2

N→δ-Ferrite+Austenite+Cr

2

N+M

23

C

6

→δ-Ferrite+Austenite+Cr

2

N+M

23

C

6

+Sigma→Austenite+Cr

2

N+M

23

C

6

+Sigma→Austenite+Cr

2

N+M

23

C

6

+Sigma+Laves→Austenite+Cr

2

N+M

23

C

6

+Sigma+Laves+ε-Cu→Austenite+Cr

2

N+M

23

C

6

+Sigma+Laves+ε-Cu+α-Ferrite→Austenite+Cr

2

N+M

23

C

6

+Sigma+Laves+ε-Cu+α-Ferrite+Pi→Austenite+M

23

C

6

+Sigma+Laves+ε-Cu+α-Ferrite+Pi.The intermetallic phases precipitated during the equilibrium transition are Sigma phase and Laves phase，in which the Sigma phase is rich in Cr and Mo， but poor in Ni and W without N， while the Laves phase is rich in Cr、Mo and W， and poor in Ni and Cu. The non-equilibrium solidification phase transition path based on the Scheil-Gulliver cooling model are as follows：Liquid → Liquid+δ-Ferrite → Liquid+δ-Ferrite+Austenite → Liquid+δ-Ferrite+Austenite+Cr

2

N → Liquid+δ-Ferrite+Austenite+Cr

2

N+M

23

C

6

→Liquid+Austenite+Cr

2

N+M

23

C

6

+Sigma→Liquid+M

6

C+Sigma.The main precipitated phase is Cr

2

N during the non-equilibrium solidification process ，the alloy element Fe is negative segregation， Cr、Ni、Mo、W、Cu、N and C are positive segregation，among which the elements Cr、Mo and W are very serious in the residual liquid phase at the end of solidification.