Aiming at the problem of wax deposition in gas well exploitation of offshore HPHT gas reservoir, the gas?liquid?solid and fluid phase equilibrium theory and method of throttling effect principle was used to reveal the wax deposition mechanism of the abnormal high temperature and high pressure gas well, and it was found that wax deposition in high temperature and high pressure gas wells mainly occurs at the position of the oil nozzle where the temperature drops sharply when the wing valve of the gas well is closed. According to the phase equilibrium theory, the phase state of the original formation fluid in HPHT gas well was recovered and characterized, and the phase state change characteristics of gas?liquid?solid three?phase fluid, wax precipitation mechanism and process were described accurately. Based on the phase equilibrium model, the wax location, fluid composition change and the influencing factors of wax deposition were analyzed. It was reasonable to explain that the mechanism of wax precipitation in HPHT gas well is the transient phase transition from gaseous to liquid and then to solid due to the rapid change of temperature and pressure, and the corresponding prevention and control strategies are put forward.