The activity coefficients in Ti 15 3 melt have been calculated. At 2 000 K,the activity coefficients in Ti 15 3 melt are all less than 1.Taking advantage of the activity coefficients,the evaporation losses of components in Ti 15 3 melt during ISM process have been studied.The calculated results show that there exists a critical vacuum degree during melting process.When the vacuum degree in the vacuum chamber surpasses that value,the evaporation of components takes on a state of free evaporation. A thermodynamic model of dissolution of oxygen in Ti 15 3 alloy is set up and the factors influencing oxygen dissolution are analyzed.The calculation results show that the oxygen partial pressure in the melting chamber is the main factor determining the oxygen dissolution in Ti alloy.A valid method was developed to decrease oxygen content and evaporation of alloy element.The calculated results are in accordance with the experimental results.With the simulation program for ISM process,the relationship between the ultimate temperature in the melt and the melting power,charge weight are set up.The change of the skull shape and size is predicted through the program too.The results show that the skull height and thickness decrease with increasing melting power and the skull is smaller than that remained in crucible after pouring.