The chloride ions and sulfate ions precipitating rates in surface layer of different mortars was experimentally measured. And the concept of precipitating rate of ions and corresponding equation in cementitious materials was proposed. Further, some factors affecting precipitating rate of ions were investigated. Results indicate that the precipitating rate of ions in surface layer of mortar can be expressed well by an exponential function of time variable. And the exponent in corresponding equation is about 0.42 in investigation scope, which cannot be influenced greatly by addition of mineral admixture or type of ions such as chloride ions or sulfate ions. However, the precipitating rate coefficient of ions changes remarkably with compositions of mortar or types of ions. Addition of mineral admixture can reduce the precipitating rate coefficient of ions.