The invention discloses a method for preparing a multi-walled carbon nanometer tube by polyacrylonitrile nanometer microsphere. Pure acrylonitrile or acrylonitrile and copolymer monomers thereof and deionized water are mixed according to a volume ratio of 1:20-1:5, are heated to a temperature between 50 and 80 DEG C after being stirred for 30 minutes at rotating speed between 300 and 800 revolutions/minute under protection of nitrogen, are added with 1-90mg/100ml of initiator to react for 1 to 12 hours, and are cooled to room temperature to directly prepare or, after being mixed with an additive, to prepare polyacrylonitrile nanometer microsphere emulsion; the polyacrylonitrile nanometer microsphere emulsion is prepared into the multi-walled carbon nanometer tube through cooling, drying, oxidizing and carbonizing. The method prepares the carbon nanometer tube with purity reaching 95 to 99 percent, even tube diameter and wall thickness by adopting the polymer nanometer microsphere as araw material. The carbon nanometer tube prepared by the method has the advantages of high purity, purification omission, good dispersibility, large-scale production and the like.