Disclosed are a drunk driving prevention system and a method of controlling the system that includes a sensor module measuring a alcohol content in the exhaled breath of a driver during a breath-checking of the driver and a control module configured to check the intoxication state of the driver from the alcohol content measured by the sensor module to determine whether the breath-checking is complete and block an engine start when the breath-checking fails, wherein the control module includes a check unit displaying breath-checking guide information through a display unit and the check unit displays the breath-checking guide information based on an engine start input of the driver.