This paper mainly focuses on the cracking problem of concrete structures, researches on the biaxial tensile strength of cement concrete. The biaxial tensile instrument is used for material test, the biaxial tensile strength formula is proposed, and the theoretic formula is deduced by Hill criterion. With the test data analysis, it shows that the calculating value and the testing value fit well. The biaxial tensile strength of cement concrete is about 15%~30% lower than its uniaxial tensile strength, putting up significant biaxial tensile strength weakening effect. This kind of weakening effect is important for designs of large-span beam, plate and shell structures under complex stress.