Abstract: Periodic solid CO2 deposition and melting phenomena happen during releasing high pressure CO2 through a safety valve from CO2 refrigeration system into atmosphere, and a longer duration of the deposition and a shorter duration of melting are advantageous for the system safety. In order to understand the characteristics of CO2 melting during releasing through the safety valve and achieve a shorter duration of melting, a transient simulation model was established. The validation of the model was done by comparison of the calculation results and experimental data. Calculation with the model showed that:(1)the downstream pressure of the safety valve increased at first and then decreased after a total blockage happened;(2)the upstream vapor quality of safety valve had a small effect on the melting time, but greatly increased the maximum downstream pressure;(3)the melting process became longer and the maximum downstream pressure increased with the increase of the length of blockage;(4) the melting time increased and the maximum downstream pressure decreased with the increase of the length of blockage and the increase of the radius of the downstream tube.