Relative humidity of microenvironment in Mogao Caves is a key factor for conservation of wall paintings. In order to realize the impact of visitation on relative humidity in Mogao Caves, the relative humidity of four relatively similar test caves in the southern area of the Mogao Grottoes were monitored regularly for ten years during 2004~2013. The relative humidity of open caves was compared to that of closed caves. The results indicated that the relative humidity fluctuation frequency in open caves was proportional to tourist numbers and that the relative humidity of open caves increased rapidly due to rain. Closing the cave could lower the impact of outdoor relative humidity significantly. Visited caves showed much higher monthly maximum relative humidity, monthly mean diurnal range of relative humidity and number of days that daily the maximum relative humidity exceeded 60%, compared with unvisited caves during high season. The changes in relative humidity were reversed when open caves were closed and closed caves were opened. Visitation would lead to higher relative humidity and higher daily fluctuations of relative humidity and thus massively elevate the risk of deterioration of wall paintings. These achievements have provided a scientific basis for preventive management and assessment of risk to the wall paintings.