Disclosed are a method and a system for monitoring the pressure of a tire. The system measures frequencies of multiple tires mounted on the vehicle, sets a first average frequency for a normal pressure state and a second average frequency for determination of a low pressure state, wherein the first average frequency is calculated by accumulating the measured frequencies for each speed interval in a stop mode and the second average frequency is calculated by accumulating the measured frequencies for each speed interval in a driving mode, calculates low-pressure probabilities for each speed interval of the tire by using the second frequency, a predetermined low-pressure reference frequency, and a predetermined frequency determination interval for determining low-pressure, and determines whether each tire is at low pressure state by aggregating the low-pressure probabilities for each speed interval of the tire.