The long-term train loads can result in problems such as strength attenuation, excessive cumulative settlement and even subsidence in sandy soil foundations and filling materials, which can jeopardize train operation safety. To understand the mechanism of the disease, it is necessary to explore the cumulative plastic deformation and pore water pressure characteristics of saturated sand under intermittent loading. Therefore, dynamic triaxial tests under continuous and intermittent loading with different dynamic amplitudes and confining pressures were performed. The test results showed that: (1) The cumulative plastic deformation-loading cycle curve of saturated sand exhibited a “zigzag” pattern. The intermittent effect led to unloading rebound and significantly reduced the accumulated plastic deformation of sand in the later loading stage, which can transform the failure mode from “destructive” to “stable” under continuous loading. (2) For plastic stability and plastic creep, the pore water pressure-loading cycle curve showed a ladder shape. In the first dynamic loading stage, the pore water pressure increased rapidly with loading cycle while drainage occurred during the intermittent stage, causing pore water pressure was dissipated and approached or equaled zero, resulting in denser sand soil. In the subsequent loading stage, the cumulative amplitude of pore water pressure decreased significantly. For the incremental failure type, the pore water pressure increased rapidly and the specimen was damaged in the first loading stage. (3) A prediction model characterizing the two-stage development of cumulative plastic strain of sand under intermittent loading was established and its prediction effect was good. (4) Intermittent effect increased the resistance of sand to plastic deformation. The cumulative plastic strain of sand was overestimated and the dynamic strength was underestimated under continuous loading. The cumulative deformation characteristics and its mechanism of saturated sand under cyclic intermittent loading can be deeply understood.