To study the problem of sand erosion through seams between geotubes, erosion stability of the backfill sand inside the dam core was carried out by a self-developed experimental apparatus. The physical model test was performed in unidirectional flow conditions and wave conditions. The transport characteristics of sand particles and scouring stability were analyzed. Experimental results show that in the condition of unidirectional flow, the process and mechanism of sand particle erosion has a “groove type” characteristic. In the condition of wave flow, arch-like structures will appear, form and collapse and correspondingly the erosion process can be divided into three stages of lashing, arching and stability. The critical length of the sand pile inside the seams between tubes was chosen as a criterion parameter to determine the erosion stability of seams between geotubes. The scouring stability in wave flow conditions can be determined by comparing the length of the seam between geotubes and the length of the sand pile inside the seam in real engineering works.