Four consolidated drained triaxial tests of an island calcareous sand were carried out to research its shear behavior,the confining pressure was 50 kPa,100 kPa,200 kPa and 400 kPa respectively. The stress and strain curves show that the calcareous sand body is shrinkage and dilatancy in turn,accompanied by the phenomenon of strain softening during the development of dilatancy. In order to further explore the reason of the strain softening of calcareous sand,then,particles smaller than 0.5mm are removed and the tests are repeated. The results show that there is no strain softening phenomenon in the calcareous sand of large particles. This paper convinces that:(1) the position of strain softening occurs relatively later,and with the increase of confining pressure,the position of the peak strength will gradually move backward;(2) the possibility of strain softening induced by particle breakage of calcareous sand under low confining pressure is very low;(3) fine particles are the key factors to induce the directional slip of flat calcareous sand;(4) the higher the confining pressure is, the higher the difficulty of the strain softening is,the peak strength is move backward gradually, accompanied by the appearance of the shear band.