Moisture is a key factor causing diseases of wall paintings. The amount and characteristics of evaporation in Cave B113 of Dunhuang Mogao Grottoes have been monitored for 5 years by the condensation method in an enclosed environment. The results show that the annual evaporation of water was 61 189.9 g and the daily evaporation was 229.2 g in Cave B113, both presenting the characteristics of sinusoidal curves. The monitoring shows that the evaporation of water in the cave increased with the rise of temperature, and vice versa. The temperature determined the concentration of evaporative water vapor, and the outflow of earth-air determined the volume of evaporative water vapor; both determined evaporation. Evaporation lasted from April to mid-December in the cave, with the largest evaporation occurring in June, July and August. Daily water evaporation rose rapidly from 7∶00 to 12∶00, and decreased from 12∶00 to 23∶00. There was also continuous water evaporation at night. It was the temperature and the amount of earth-air activity that dominated and controlled the amount of evaporation in the cave. The revelation of the water evaporation mechanism in caves provides a theoretical and scientific basis for the regulation of the micro-environment and the preventive conservation of wall paintings in the future.