The present study investigated the physicochemical characteristics of alkali-activated fly ash/slag (AAFS) modified by MgO and the influence of MgO on the chloride penetration resistance of AAFS. MgO was added to replace the fly ash and slag at dosages of 0%, 5% and 10% by weight of the binder. Samples were immersed in a 10 wt% sodium chloride solution for 91 days. The results showed that the formation of hydrotalcite was promoted by MgO incorporation. Hydrotalcite was found to be effective in improving chloride penetration resistance. However, there exists a threshold level upon which the amount of hydrotalcite may not further increase the chloride penetration resistance. In addition, the adsorption of chloride in AAFS can be seen not only as the result of effects of hydrotalcite but also as the combined effect of the pore network structure and pore solution chemistry.