Control of the water level in the drum of the boiler system is a critical operation consideration, because asymmetric dynamics of water level measured leads to measuring errors. This phenomenon is testified by field data from a 400t/h drum boiler with natural circulation in a power plant of China. Some equations are developed and several related factors are focused on, and two of which become critical ones accounting for asymmetry of measured value. With the study of structure of distribution pipes in drum, it is found that feed-water flows distributed on the opposite ends of drum are obviously asymmetric, where measurement systems are positioned. When more feed-water is supplied at one end of drum, the rate of steam in water and water temperature become lower, and measured level get higher. Through the simulation, the important factors for asymmetric of water level are found out, which is necessary to minimize the measuring errosrs.