On the basis of the failure mechanism of concrete under tension and compression, the difference between tensile and compressive strengths of full-grade and wet-sieving concretes is investigated in this paper. Considering the experimental condition nowadays, the reason that some experiments obtain the ratio of compressive strength of full-grade and wet-sieving concretes almost approaching 1, moreover more than 1, is analyzed. With the influence of raw materials, proportioning variation, mould flatness and shaping method, some errors may be led into the experiments of full-grade concrete test. The shaping method of concrete should be improved to reflect the real large size concrete properties by full-grade test in lab.