Abstract: On the basis of the research on the behavior of tensile strain stress relationship of compacted soil in the past few decades, a new strain controlled soil beam bending instrument was invented,and more than 200 compacted soil samples including loess soil from northwest, black soil from northeast, red soil from southwest of China were tested by the new device. Through the experimental study on the compacted soil sample, the strain process of the soil sample is divided into four phases and the method to gain the limit value of tensile strain under bending was given in this paper.The effect of strain rate, the water content and the dry unit weight are also considered,besides, the range of the limit value of tensile strain under bending is obtained in the paper.