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A combination of experimental studies and numerical simulations is employed to probe into the transient heat transfer phenomenon during rapid depressurization of supercritical fluid in a cylindrical vessel connected to atmosphere via a sudden valve on the top. The effects of initial conditions on heat transfer characteristics are studied, and the morphological characteristics of supercritical fluid are analyzed. Results show: (i) there is a strong re-circulation inside the supercritical fluid with a sudden pressure drop. The heat transfer process is divided into four stages: the initial depressurization stage, the rapid evaporation stage, the nucleate boiling stage and the surface evaporation stage; (ii) under the subcritical state, a higher temperature contributes to a faster temperature drop rate and a shorter fast evaporation time. Moreover, a higher initial pressure leads to a lower equilibrium temperature and a shorter time it takes to reach the equilibrium temperature; (iii) differences are presented in terms of the effect of initial pressure in subcritical and supercritical zones on temperature drop rate with a turn near the critical point. A higher initial pressure gives rise to a higher equilibrium temperature, a slower temperature drop rate and the gradual increasing fast evaporation time; (iv) the experimental data are in good agreement with the calculated results, with a maximum error of 10%.