The critical characteristics, condensation and supercoolingphase transition processes of real gas are derived in terms of the van der Waals equation. The results of analysis indicate that the stronger attraction and the weaker repulsion between molecules correspond to the higher phase transition temperature. The supercooled gas, due to the pressure resistance, below the critical temperature is metastable, which requires an external induction to overcome the resistance to conduct phase transition. To reduce the temperature will decrease the resistance and increase the volume shrinkage from supercooling state to phase transition point. The faster the temperature drops, the faster the phase transition of the supercooled gas will be. The influence of pressure on the gasliquid coexistence line has a discrepancy of a correction factor from that in the ideal gas.