Abstract: Average foams life, $\bar τ$ has been presented as criterion to evaluate the foaming behavior in molten slag. Carbonate minerals as foamer, molten salt system of Na₂B₄O₇-CaO-MgO is foamed by the gas from the heat decompostion of the foamer in laborstory. The experimental results show that the particle size and foamer type have effects upon average foams life of slag, and these effects are dependent on the behavior of heat decomposition of carbonate. Within a certain extent, the variation of concentration of CaO and MgO in molten slag both varies the physical properties of melt, and directly influences the decomposition rate and of carbonate and the bubble size of gas from decompositions, so it plays important pole in foaming and foams stability of slag.