Based on the activity coefficient calculation module, the saturated pressures of Ti, Al and Nb elements in molten Ti-13Al-29Nb-2.5Mo (mass fraction) at different melting temperatures were calculated. The saturated pressure of element Al in the melting temperature is as big as 17 and 3 070 times of those of Ti and Nb elements respectively. The mass loss rates of Al, Ti, Nb in the melt during the process were also calculated. The mass loss rate of Al is as big as 43 and 144 times of those of Ti, Nb. Based on the evaporation loss rate, the evaporation loss quantity of the elements and the alloy chemical composition after the ISM process was also calculated. Because the mass loss rates of Al, Ti and Nb are different, the ultimate alloy chemical composition deviated from its original composition after the melting process. Analysis shows that properly decreasing the melting temperature and/or increasing the chamber pressure can decrease the composition deviation. This article will give some valuable guidance to the alloy composition controlling during ISM process.