Abstract: The effects of expansive agent (EXP), steel fiber (SF) and different blending ratio of steel fiber and polyvinyl alcohol (PVA) fiber on mechanical properties of self-compacted concrete were investigated, and the modification mechanism was analyzed from the perspective of mesoscopic and microscopic pore structures on the surface of concrete test blocks by image processing. The results show that the influence of expansive agent on the early compressive strength is related to the specific fiber content, and the simultaneous addition of expansive agent and steel fiber can improve the mechanical properties of self-compacting concrete. When 7% or 10% expansive agent and 0.20% steel fiber are added, the structure of meso pores and micro pores on the surface of self-compacting concrete can be improved effectively. The compressive strength of 7 d is negatively correlated with the average pore diameter and the roundness of 7 d meso pores, but positively correlated with the roundness of 7 d micro pores. The 28 d compressive strength is negatively correlated with the porosity of 28 d meso pores and micro pores.