To investigate the mechanical properties of alkali-activated slag ceramsite concrete blocks after exposure to high temperature, compression tests were conducted on 48 alkali-activated slag ceramsite concrete blocks with two strength grades, MU10 and MU20, and all the specimens were exposed to normal temperature and a temperature history of 400 ℃, 600 ℃, 700 ℃, 800 ℃, 900 ℃, 1 000 ℃, and 1 100 ℃. The results show that the compressive strengths of MU10 and MU20 ceramsite concrete blocks exhibited a linear decrease with the increase of the heating temperature from 20 ℃ to 1 100 ℃. Based on the test results, the formulas were constructed to calculate the compressive strengths of the alkali-activated slag ceramsite concrete blocks after exposure to high temperature in term of the heating temperatures.