The invention relates to a method for preparing high-quality graphene powder at low cost and large scale. The method comprises the following steps of: adding graphite to mixed solution containing an oxidizing agent and an intercalator, stirring evenly and then performing ultrasonic treatment, and simultaneously, continuously introducing He to form a graphite intercalation compound intercalated with the intercalator and He gas molecules; next, filtering, washing and drying, and performing thermal treatment in air to realize the first time of stripping of the graphite intercalation compound; later, dispersing the graphite intercalation compound in an organic solvent and performing ultrasonic treatment again under the condition of continuously introducing He; and then centrifuging and removing precipitate, and filtering, washing and drying the solution of the upper layer, thereby obtaining the graphene powder. The method provided by the invention is safe and environment-friendly, simple to operate and suitable for large-scale production; the prepared graphene has few defects and good electrical conductivity.