Abstract: Based on the characteristics of complicated composition in diesel，equation of state was applied to the description of the liquid phase，and regular solution theory was applied to the description of the solid phase for the sake of calculation of the compositions of the phases of liquid and solid in equilibrium. For the first time，non-n-alkane composition was regarded as a pseudo-component to simplify calculation. The thermodynamic model can successfully predict the temperature of wax appearance，which is close to the cloud point. The trend that every n-alkane in diesel was settled was studied by means of calculated compositions of solid phase and solid phase. The proportion of heavy n-alkane in solid was still small despite the stronger settling trend of heavy n-alkanes than light n-alkanes, because the proportion of heavy n-alkane in the original diesel was small. n-Eicosene，n-octadecane and n-docosane were most easily precipitated in diesel. The average carbon number of n-alkane in settled solid became less with increasing temperature. The phenomena that the settled waxy crystals entrapped significant amounts of liquid could be explained by calculated results obtained from thermodynamic model.