The present invention provides a system and method for preparing high-purity vanadium pentoxide powder. Industrial grade vanadium pentoxide is converted to vanadium oxytrichloride by low temperature fluidizing chlorination, wherein chlorinating gas is preheated via heat exchange between fluidizing gas and chlorination flue gas, and an appropriate amount of air is added to enable a part of carbon powder to combust so as to achieve a balanced heat supply during the chlorination, thereby increasing the efficiency of chlorination and ensuring good selectivity in low temperature chlorination. The vanadium oxytrichloride is subjected to purification by rectification, ammonium salt precipitation and fluidized calcination, thereby obtaining high-purity vanadium pentoxide, wherein the ammonia gas produced during calcination is condensed and then recycled for ammonium salt precipitation. The system and method have advantages of favorable adaptability to raw material, less pollution, low energy consumption in production, low operation cost, stable product quality, etc.