Activated red mud and slag were used as the main raw material to prepare red mud geopolymer cement（RMPC）. The effects of slag dosage， activator （sodium silicate） modulus and its dosage on mechanical properties and polymerization mechanism of RMPC were studied. The results show that slag can significantly improve the mechanical properties of RMPC mortar. When the dosage of slag is 40%， the compressive strength of RMPC mortar is on the highest. When the modulus of sodium silicate is 1.5 or 1.2， the compressive strength and flexural strength of RMPC mortar are on the best. When the modulus of sodium silicate is 1.5， the compressive strength of RMPC mortar increases with the increase of the dosage of sodium silicate， and the flexural strength is on the highest when the dosage of sodium silicate is 20%. The active silicon and aluminum components in red mud and slag participate in the geological polymerization reaction and hydration hardening process under the action of sodium silicate， and form the geopolymer structure with the framework of zeolite geopolymer and C-（A）-S-H gel.