The Guiling Island wharf project in Shanwei City，Guangdong Province adopts clay-sand mixed soil foundation. The results of the undrained shear tests with different sand contents show that：1）The fractal dimension of mixed soil decreases rapidly with the increase of sand content. 2）The shear strength characteristics gradually change from hardening type to softening type. 3）The fractal dimension of 2.709 3（sand content of 50%）is the limit value for the obvious change of mechanical properties of clay sand mixed soil，and the mechanical properties of two sides show great differences. 4）The angle of internal friction increases and the cohesion decreases. Combined with the foundation bearing capacity of the empirical formula，the actual material and other factors to determine the sand content of 30%～40%. 12 monitoring points are selected to review the results.