Under cyclic loading, sand will undergo a solid-liquid phase transition during the liquefaction. This study utilizes discrete element method (DEM) to investigate the stage characteristics of sand macroscopic stress-strain response during the solid-liquid phase transition. The microscopic mechanism of sand solid-liquid phase transition is elucidated from the perspective of contact network. The results indicate that based on the sand flowability, the liquefaction process can be divided into solid phase, solid-liquid transition phase, and liquid phase stages. The strong contact network within the sand is the primary contributor to its effective stress, and the degradation of the originally well-connected strong contact network are the reasons for the sand solid-liquid phase transition. A parameter xi c has been proposed to measure the connectivity of the strong contact network. The weak contacts between particles dominates the sliding and rolling between particles, which is the reason for the macroscopic deformation and flow of sand.