The invention belongs to the technical field of steel refining, and particularly relates to a device and a method for single snorkel vacuum degassing refining of molten steel by employing bottom injection. The device disclosed by the invention comprise an air supply system, a powder spray system, a single snorkel vacuum degassing refining furnace, wherein an airflow distributor on the air supply system is branched into three gas circuits which are connected with the powder spray system; a powder airflow delivery pipe in the powder spray system is connected with a bottom powder injection element; the bottom powder injection element is built at the bottom of a ladle furnace of the single snorkel vacuum degassing refining furnace; and an opening is formed in the lower end of a vacuum chamber of the single snorkel refining furnace and is inserted into a steel ladle molten pool. The method for single snorkel vacuum degassing refining of molten steel by employing bottom injection comprises the following steps: firstly, blowing a gas and stirring molten steel; blowing powder into the steel ladle through the bottom powder injection element under the transportation of a carrier gas flow; and after the blowing time is ended, entering the next work circle. The device and the method are easy to achieve, short in production cycle, and high in production capacity; equipment investment and operation costs on an overhead bunker and a wire feeding mechanism are reduced; the technological operation is simple; and energy conservation and emission reduction are facilitated.