Abstract: In order to clarify the influences of cracks on the mechanical properties of concrete under the uniaxial tensile loads and the Kaiser effect of the acoustic emission under variable amplitude cyclic tension loads, three kinds of axial tensile specimens with different crack lengths have been tested to study the influences of the initial crack length on the axial tensile strength, fracture toughness, elastic modulus and the Kaiser effect. The research results indicate that the elastic modulus remains constant, the fracture toughness increases gradually, and the tensile strength decreases linearly with the increase in the cracking length of the concrete specimens. Under the cyclic loads with variable amplitude, the concrete specimens show obvious Kaiser effect, and the Kaiser effect is gradually weakened with the increase of the load level. When the crack-depth ratio is less than 0.4, the Kaiser effect increases with the increase of the initial crack length of the concrete specimens. The Kaiser effect is essentially determined by the degree of the internal damage of concrete specimens. In addition, the strength of concrete specimens under the axial tension will affect the activation intensity of the acoustic emission signal at the low load level. Based on this, the strength change of the concrete specimens can be judged.