Abstract: The material in the system from a phase transition to another phase process called phase transition,phase transition balance after they say the system to achieve the phase equilibrium.The phase equilibrium method of temperature,pressure and the phase composition of the relationship with geometric figure said out,called phase diagram equilibrium diagram,referred to as the phase diagram.Another method is to use thermodynamic basic formula is deduced the system temperature,pressure and the phase composition of the quantitative relation between,namely Clapeyron equation.Therefore choose research unit subsystem two phase equilibrium,temperature T and pressure between p function relation-clapeyron equation,and then illustrates unit subsystem of the phase diagram and under different conditions clapeyron equation of the specific application.