The geotechnical test can't be applied to sand as the original sample is difficulty to collected by drilling, so the cohesive strength C of sand can't be acquired in geotechnical test. Generally speaking, according to the Mohr–Coulomb law, C=0. From the numerical point of view, when the slope angle is greater than the internal friction angle, the global stability of slop can’t meet the requirements of standard. However, that’s not the case in fact. In practice, the slope angle can be determined by means of analogy. Although successful experiences have obtained, there still exist some potential hazards. In this paper, we have made some studies on the cohesive strength of sand based on the control of instability of a deep foundation in Xiang'an District, Xiamen, China.