A liquefied petroleum gas (LPG) filling system of a bi-fuel vehicle is provided. The LPG filling system may be configured for reducing the temperature and pressure in an LPG bombe for storing LPG in the case in which the external temperature is very high, e.g. in the hot season, whereby it is possible to easily refill the LPG bombe with LPG. The LPG filling system is configured to cool the inside of an LPG bombe and to reduce the vapor pressure of LPG by supplying some gasoline from a gasoline tank into the LPG bombe using the fact that the temperature of gasoline in the gasoline tank is lower than the temperature of LPG in the LPG bombe, whereby it is possible to easily refill the LPG bombe with LPG even in the case in which the external temperature is very high, e.g. in the hot season.