Influence of temperature increase rates (5℃/min, 10℃/min), duration at target temperatures (1h, 2h) and cooling methods (natural cooling, water cooling) on residual compressive strength of alkali activated slag concretes after high temperature has been studied in this paper. Microstructural change of alkali activated slag matrix with temperature elevation was also studied to analyse the change in strength. The results show that when the higher rate was used, within 400oC, the influence of thermal stress was not sufficient, a shorter duration in heat could remain the strength of the concretes at a considerable level; with temperature elevation to 800oC, residual strength decreased dramatically with the increase of thermal stress; afterwards, thermal stress has achieved a certain extent, a shorter duration in heat remained the strength. Increase in duration at target temperature has intensified deterioration of alkali activated slag matrix and matrix-aggregates ITZ, consequently decreased the residual strength. Water cooling has lowered the temperature of C-S-H hydrates to decompose, as well as resulted in thermal stress in concretes, consequently limited the remain of the residual strength.