The formation of ordered ice nanotubes inside carbon nanotubes was investigated through molecular dynamics simulations. Water confined in carbon nanotubes with diameters ranging from 1.1 nm to 1.4 nm and under axial pressures of 50–500 MPa exhibited various ice phases, including hexagonal and heptagonal ice nanotubes, as well as square and pentagonal ice nanotubes. The simulations revealed a first-order freezing transition to hexagonal and heptagonal ice nanotubes and a continuous phase transformation into square or pentagonal ice nanotubes. The results suggest the existence of a solid-liquid critical point, where the distinction between solid and liquid phases disappears. The structural analysis showed that water molecules form quasi-one-dimensional (Q1D) structures inside the nanotubes, leading to new ice phases not observed in bulk ice. The phase behavior of water in the (14,14) SWCN was examined, revealing a solid-liquid critical point. The phase diagram of water in the (14,14) SWCN indicates that the square liquid and pentagonal ice nanotube phases are separated by a first-order phase boundary. The critical point, beyond which the pentagonal ice nanotube and square liquid lose their identities, is likely located between 280 and 300 K and 200 and 300 MPa. The study provides insights into the unique phase behavior of water in confined spaces and the potential for new ice phases in nanoscale environments.The formation of ordered ice nanotubes inside carbon nanotubes was investigated through molecular dynamics simulations. Water confined in carbon nanotubes with diameters ranging from 1.1 nm to 1.4 nm and under axial pressures of 50–500 MPa exhibited various ice phases, including hexagonal and heptagonal ice nanotubes, as well as square and pentagonal ice nanotubes. The simulations revealed a first-order freezing transition to hexagonal and heptagonal ice nanotubes and a continuous phase transformation into square or pentagonal ice nanotubes. The results suggest the existence of a solid-liquid critical point, where the distinction between solid and liquid phases disappears. The structural analysis showed that water molecules form quasi-one-dimensional (Q1D) structures inside the nanotubes, leading to new ice phases not observed in bulk ice. The phase behavior of water in the (14,14) SWCN was examined, revealing a solid-liquid critical point. The phase diagram of water in the (14,14) SWCN indicates that the square liquid and pentagonal ice nanotube phases are separated by a first-order phase boundary. The critical point, beyond which the pentagonal ice nanotube and square liquid lose their identities, is likely located between 280 and 300 K and 200 and 300 MPa. The study provides insights into the unique phase behavior of water in confined spaces and the potential for new ice phases in nanoscale environments.