We take C25 natural sand concrete as the research object and study the effect of oyster shell content on the compressive strength of concrete at the same strength grade and with different replacement rates. The oyster shell replacement rate with the maximum compressive strength of concrete at 28 d is obtained and compared with natural sand concrete， and its mechanism is analyzed. Meanwhile， the failure mode of concrete compressive strength test is further analyzed. The results show that： the compressive strength of oyster shell concrete decreases with the increase of oyster shell replacement rate. In general， the strength of concrete replaced by oyster shell meets the requirements for strength grade standards， but the compressive strength of oyster shell concrete is lower than that of natural sand concrete. The failure mode of oyster shell concrete is related to the oyster shell content， and the less oyster shell content， the higher the damage degree of concrete.