Abstract: The pressure cell installed under road is required to have the properties of durability and reliability for long-term usage. Usually it is considered unable to measure dynamic pressure, the distribution of dynamic pressures on railway subbase surface excited by train can be measured by the string-type pressure cells with a frequency detector by means of the probability-sampling method suggested in this paper. According to the philosophy of probability-sampling the probabilistic distribution (frequency distribution) curves of dynamic pressures are plotted to show the dynamic pressure distribution along the longitudinal direction of the railway approximately. The principle and method were successfully used and examined in the research on long-term monitoring of subgrade stability of the Qinghai-Tibet Railway in the Qarhan Salt-Lake region.