A heterogeneous gravel bed is the main stratum for wall paintings of Mogao Grottoes, but it is still not clear how the heterogeneity of the stratum plays in the transport of water and salt in it. By three-dimensional high-density electrical resistivity tomographic monitoring, conducted in 1 m×1 m and 2 m×2 m regions at the surface of the west side wall of Cave 108, we found the spatial distributions of wall resistivity were heterogeneous in both regions. The spatial position, size and shape of the low-resistivity regions all changed with the time. This clearly indicates that there exists important water saturation and desaturation process (i.e. water redistribution process) in the heterogeneous gravel strata forming the painting wall. It is this repeated water saturation and desaturation process that drives the water to flow in the wall and causes the salt to accumulate in specific regions. The results of the study have a wider significance for fully understanding the water and salt transport mechanism in the walls of Mogao Grottoes, as well as reducing and preventing the salt-related damage to the wall paintings.