The invention discloses a system for determining the explosion suppression performance of liquid fuel vapor by water vapor. The system comprises an explosion bin (1), a computer, a main control system, and a first vacuumizing device (7) connected with the main control system, a gas circulation device (8), a liquid fuel vapor supplying device (4), a dry air supplying device (5), a water vapor supplying device (9), an ignition device and a constant-temperature water supplying device, wherein the liquid fuel supplying device (4), the dry air supplying device (5), the water vapor supplying device (9) and the first vacuumizing device (7) are all communicated with the explosion bin (1); the explosion bin (1) is internally provided with ignition electrodes (6) and an explosion bin pressure sensing device; the main control system is connected with the computer. The invention also discloses a method for determining the explosion suppression performance of liquid fuel vapor by water vapor. The system for determining the explosion suppression performance of liquid fuel vapor by water vapor has the advantages of determination accuracy, safety and reliability.