Abstract: The caverns No. 1 to No. 5 of Longyou cavern group had been fully filled with water for many hundreds years. In 1992, the water was pumped out d for tourism. Because of the dramtic changes of the condition and the alternation bettween drying and wetting, the rock mass weathering and the damages have been substantial. Such as, the rock pillar No. 3-2 of the cavern No. 3 has developed many cracks. This pillar has to be reinforced immediately. To provide the basis for the pillar reinforcement, it is needed to fully understand the situation of the deformation development in the pillar. This paper presents the monitoring results of the widths and the lengths of the cracks with the jointmeters of JSYCB-73 series and steel ruler. On the basic of the data obtained from the monitoring and the geology conditions, this paper further analyzes the causes of the cracking. The following findings are obtained: (1) part of the weights of concrete materials that are used to reinforce the other part of the cavern increased the load on the pillar；(2) the leakage of the cavern, the alternation bettween drying and wetting and the changes of the temperature enlarged the weathering, which reduced the pillar strength；(3) the two above factors made the growth of cracks in the pillar. Based on the findings, taking into account the cultural relics and the tourism, this paper suggests the protection measures. They include the water-control, unloading the load, installing rock bolts and hoops. The monitored data after the pillar reinforcments show that the cracks has stopped developping，and the pillar has been stable.