Cement based composite cementitious material containing steel slag used in road has been prepared to deal with the current situation that the transportation carbon emission increased year by year. In this material, 40% cement has been replaced by equivalent steel slag, which has the ability of CO2 sequestration. This paper studied the CO2 sequestration effect and the mechanical properties of the pure cement, the pure steel slag, and the cement based composite cementitious materials containing steel slag. It has been shown that the cement based composite cementitious materials containing steel slag have excellent CO2 sequestration property. The mass fraction growth rate of carbon reached 10.86% after 1 hour carbonation experiment, the value between which of pure cement and pure steel slag, and the compressive strength of the composite cementitious materials at 28-day age can reach 45.3MPa, meeting the requirements of road.