Despite the simplicity of the vortex tube’s geometry，the fundamental mechanism underlying the energy separation effect is still elusive due in part to the complexity of flow behavior inside the tube.In order to investigate its fundamental mechanism，a three-dimensional simulation was performed to predict the flow field and temperature distribution within a vortex tube.The results demonstrated that the large pressure gradient in the radial and axial directions was the main reason for the separation of hot and cool flow.Radial centrifugal force led to different static pressures at different positions along the axis.Inner flow experienced an adiabatic expansion process while peripheral flow experienced an adiabatic compression process.Due to gradient distribution of static temperature from axial center to periphery at different positions along the axis and re-circulating flow，heat trasfer and mass transfer occurred between inner and peripheral flow.Meanwhile，the numerical data and experimental results were compared and good agreement was obtained.