Sand particles in oil and gas fluid can accelerate CO2 corrosion of metallic materials due to the synergism of erosion and corrosion. The erosion-corrosion damage mechanism gets more complex in sand particles and CO2 coexisting environment than that in single sand or CO2 environment. Usually sand particles can mechanically damage the metallic materials or corrosion scale to accelerate corrosion. Therefore it is important to obtain the erosion-corrosion regularity and mechanism of metallic materials in sand containing high temperature and high pressure (HTHP) CO2 environment in order to control the damage of metallic materials. In this paper the erosion-corrosion experiment was conducted with X70 pipeline steel as a tested material by rotating rage. Silica sand was used in the experiment. The damage regularity and mechanism of X70 pipeline steel in HTHP CO2 environments were analyzed and discussed in detail. The electrochemical behavior was also researched to understand the effect of sand particles on corrosion scale. The effects of concentration and size particles of silica sand on corrosion rates and morphologies were given at CO2 partial pressures. Further the erosion-corrosion mechanism was brought forward with the electrochemical measurement and micro morphologies of X70 steels covered with corrosion scale.