The invention provides a feeding device used for synthesizing InP polycrystals and a method of synthesizing the InP polycrystals. The feeding device used for synthesizing the InP polycrystals comprises a feeding bottle, a cover body, a protective gas supply bottle and a conveying pipe; the feeding bottle has a receiving cavity which is used for holding a red phosphorus material and an indium material and has a closed end and an opening end; the cover body can cover the opening end of the feeding bottle, and the cover body is provided with a gas discharging hole which communicates with the exteriors of the receiving cavity and the feeding bottle; the protective gas supply bottle is used for holding a protective gas which does not react with the red phosphorus material, and the protective gas supply bottle is provided with a gas outlet; and one end of the conveying pipe communicates with the gas outlet of the protective gas supply bottle, and the other end passes through the cover body and extends into the receiving cavity of the feeding bottle to convey a protective gas to the receiving cavity of the feeding bottle. According to the feeding device used for synthesizing the InP polycrystals provided by the invention, the risk of spontaneous combustion of the red phosphorus material in air containing oxygen due to friction and collision is effectively avoided, and the pollution tothe subsequently formed InP polycrystals caused by of an oxide formed by combustion of the red phosphorus material and the air is further avoided.