In this paper the modelling and numerical simulation of the freezing and thawing processes of water under high hydrostatic pressure up to 200 MPa is presented. The system of equation consists of the conservation laws of mass, momentum and energy. Moreover, it contains also the equation of phase change and the equations of state for water and ice. In order to model the phase transitions the enthalpy-porosity method is used. The finite-volume method is used to solve the system of equations and to simulate different cases of phase transitions. After analysing the results from our numerical simulations it can be seen that the fluid flow in a vessel at high pressure differs from that at normal pressure. The spatial ice developing depends more strongly on convection than on conduction in larger high pressure chambers. Very good agreement between results from numerical simulation and experiment is obtained.