To explore the subsurface erosion characteristics of sand with varying particle morphologies under unsteady flow conditions, three types of coarse-grained sand with distinct particle morphologies, namely quartz sand from Fujian Province, river sand from the Yangtze River Delta, and weathered sand from Anhui Province, were selected for investigation. Three key particle morphology parameters were utilized, including aspect ratio, roundness, and roughness. An independently designed apparatus capable of applying diverse hydraulic loading methods for subsurface erosion tests was employed to assess the anti-subsurface erosion performance of sand with different particle morphologies. The results demonstrate that under various water head conditions, the anti-erosion coefficient of the samples is negatively correlated with both the void ratio and the particle morphology parameters. Specifically, as the void ratio increases, the sample’s density decreases, thereby reducing its anti-subsurface erosion performance. Moreover, as the morphology of the coarse-grained sand is closer to a standard sphere, the anti-subsurface erosion performance of the samples gets lower. Under identical void ratio, coarse-grained sand morphology, and gradation, the anti-erosion coefficient under the constant water head condition is greater than that under the cyclic water head condition. In other words, the anti-erosion coefficient of the samples will be reduced under the cyclic water head condition, and subsurface erosion is more likely to occur under the cyclic water head condition.