The influences of slag on the expansion of alite-calcium barium sulphoaluminate cement were studied. The experimental results indicated that the formation of ettringite and Ba-bearing ettringite are the main expansion sources of alite-calcium barium sulphoaluminate cement clinker with only gypsum. In addition to the ettringite and Ba-bearing ettringite, the Mg(OH)2 crystal formation was another probably the expansion source. The expansion rate of alite-calcium barium sulphoaluminate cement increased with slag content rising. Even if the content of slag in cement arrived at 20%, the soundness of alite-calcium sulphoaluminate cement its expansion was still in the safety range.